diff --git a/csharp/src/AddressBook/Addressbook.cs b/csharp/src/AddressBook/Addressbook.cs
index d3e1ea95344f..c05125c629cd 100644
--- a/csharp/src/AddressBook/Addressbook.cs
+++ b/csharp/src/AddressBook/Addressbook.cs
@@ -260,7 +260,9 @@ public sealed partial class Person : pb::IMessage<Person> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -440,7 +442,9 @@ public sealed partial class PhoneNumber : pb::IMessage<PhoneNumber> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 10: {
                 Number = input.ReadString();
@@ -573,7 +577,9 @@ public sealed partial class AddressBook : pb::IMessage<AddressBook> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             people_.AddEntriesFrom(input, _repeated_people_codec);
diff --git a/csharp/src/Google.Protobuf.Conformance/Conformance.cs b/csharp/src/Google.Protobuf.Conformance/Conformance.cs
index cd9b77abe5fc..1642669e92ca 100644
--- a/csharp/src/Google.Protobuf.Conformance/Conformance.cs
+++ b/csharp/src/Google.Protobuf.Conformance/Conformance.cs
@@ -343,7 +343,9 @@ public enum PayloadOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             ProtobufPayload = input.ReadBytes();
@@ -683,7 +685,9 @@ public enum ResultOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             ParseError = input.ReadString();
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs b/csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs
index 6d3cd026d112..2844ca62ce4e 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs
@@ -541,7 +541,9 @@ public sealed partial class TestMap : pb::IMessage<TestMap> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             mapInt32Int32_.AddEntriesFrom(input, _map_mapInt32Int32_codec);
@@ -737,7 +739,9 @@ public sealed partial class TestMapSubmessage : pb::IMessage<TestMapSubmessage>
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (testMap_ == null) {
@@ -861,7 +865,9 @@ public sealed partial class TestMessageMap : pb::IMessage<TestMessageMap> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             mapInt32Message_.AddEntriesFrom(input, _map_mapInt32Message_codec);
@@ -1001,7 +1007,9 @@ public sealed partial class TestSameTypeMap : pb::IMessage<TestSameTypeMap> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             map1_.AddEntriesFrom(input, _map_map1_codec);
@@ -1350,7 +1358,9 @@ public sealed partial class TestArenaMap : pb::IMessage<TestArenaMap> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             mapInt32Int32_.AddEntriesFrom(input, _map_mapInt32Int32_codec);
@@ -1531,7 +1541,9 @@ public sealed partial class MessageContainingEnumCalledType : pb::IMessage<Messa
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             type_.AddEntriesFrom(input, _map_type_codec);
@@ -1666,7 +1678,9 @@ public sealed partial class MessageContainingMapCalledEntry : pb::IMessage<Messa
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             entry_.AddEntriesFrom(input, _map_entry_codec);
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs b/csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs
index d74422b59b32..683dc41d0e51 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs
@@ -2867,7 +2867,9 @@ public enum OneofFieldOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             OptionalInt32 = input.ReadInt32();
@@ -3582,7 +3584,9 @@ public sealed partial class NestedMessage : pb::IMessage<NestedMessage> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 A = input.ReadInt32();
@@ -3723,7 +3727,9 @@ public sealed partial class ForeignMessage : pb::IMessage<ForeignMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             C = input.ReadInt32();
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs
index 2a9efe55fff1..9b9ffe256005 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs
@@ -352,7 +352,9 @@ public enum AnOneofOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Field1 = input.ReadString();
@@ -478,7 +480,9 @@ public sealed partial class CustomOptionFooRequest : pb::IMessage<CustomOptionFo
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -579,7 +583,9 @@ public sealed partial class CustomOptionFooResponse : pb::IMessage<CustomOptionF
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -680,7 +686,9 @@ public sealed partial class CustomOptionFooClientMessage : pb::IMessage<CustomOp
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -781,7 +789,9 @@ public sealed partial class CustomOptionFooServerMessage : pb::IMessage<CustomOp
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -882,7 +892,9 @@ public sealed partial class DummyMessageContainingEnum : pb::IMessage<DummyMessa
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -996,7 +1008,9 @@ public sealed partial class DummyMessageInvalidAsOptionType : pb::IMessage<Dummy
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -1097,7 +1111,9 @@ public sealed partial class CustomOptionMinIntegerValues : pb::IMessage<CustomOp
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -1198,7 +1214,9 @@ public sealed partial class CustomOptionMaxIntegerValues : pb::IMessage<CustomOp
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -1299,7 +1317,9 @@ public sealed partial class CustomOptionOtherValues : pb::IMessage<CustomOptionO
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -1400,7 +1420,9 @@ public sealed partial class SettingRealsFromPositiveInts : pb::IMessage<SettingR
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -1501,7 +1523,9 @@ public sealed partial class SettingRealsFromNegativeInts : pb::IMessage<SettingR
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -1690,7 +1714,9 @@ public sealed partial class ComplexOptionType1 : pb::IMessage<ComplexOptionType1
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Foo = input.ReadInt32();
@@ -1902,7 +1928,9 @@ public sealed partial class ComplexOptionType2 : pb::IMessage<ComplexOptionType2
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (bar_ == null) {
@@ -2051,7 +2079,9 @@ public sealed partial class ComplexOptionType4 : pb::IMessage<ComplexOptionType4
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 Waldo = input.ReadInt32();
@@ -2185,7 +2215,9 @@ public sealed partial class ComplexOptionType3 : pb::IMessage<ComplexOptionType3
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Qux = input.ReadInt32();
@@ -2293,7 +2325,9 @@ public sealed partial class VariousComplexOptions : pb::IMessage<VariousComplexO
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -2475,7 +2509,9 @@ public sealed partial class Aggregate : pb::IMessage<Aggregate> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             I = input.ReadInt32();
@@ -2615,7 +2651,9 @@ public sealed partial class AggregateMessage : pb::IMessage<AggregateMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Fieldname = input.ReadInt32();
@@ -2723,7 +2761,9 @@ public sealed partial class NestedOptionType : pb::IMessage<NestedOptionType> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -2855,7 +2895,9 @@ public sealed partial class NestedMessage : pb::IMessage<NestedMessage> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 NestedField = input.ReadInt32();
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs
index 6bf97151715e..931dc2eb03a0 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs
@@ -167,7 +167,9 @@ public sealed partial class ImportMessage : pb::IMessage<ImportMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             D = input.ReadInt32();
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs
index 97d181aff48c..fee0120b42d1 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs
@@ -155,7 +155,9 @@ public sealed partial class PublicImportMessage : pb::IMessage<PublicImportMessa
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             E = input.ReadInt32();
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs
index f27ab6407497..f8296b0fabb4 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs
@@ -181,7 +181,9 @@ public sealed partial class Issue307 : pb::IMessage<Issue307> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -284,7 +286,9 @@ public sealed partial class NestedOnce : pb::IMessage<NestedOnce> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
             }
           }
@@ -387,7 +391,9 @@ public sealed partial class NestedTwice : pb::IMessage<NestedTwice> {
               while ((tag = input.ReadTag()) != 0) {
                 switch(tag) {
                   default:
-                    _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                    if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                      return;
+                    }
                     break;
                 }
               }
@@ -554,7 +560,9 @@ public sealed partial class NegativeEnumMessage : pb::IMessage<NegativeEnumMessa
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Value = (global::UnitTest.Issues.TestProtos.NegativeEnum) input.ReadEnum();
@@ -669,7 +677,9 @@ public sealed partial class DeprecatedChild : pb::IMessage<DeprecatedChild> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -899,7 +909,9 @@ public sealed partial class DeprecatedFieldsMessage : pb::IMessage<DeprecatedFie
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             PrimitiveValue = input.ReadInt32();
@@ -1056,7 +1068,9 @@ public sealed partial class ItemField : pb::IMessage<ItemField> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Item = input.ReadInt32();
@@ -1209,7 +1223,9 @@ public sealed partial class ReservedNames : pb::IMessage<ReservedNames> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Types_ = input.ReadInt32();
@@ -1323,7 +1339,9 @@ public sealed partial class SomeNestedType : pb::IMessage<SomeNestedType> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
             }
           }
@@ -1647,7 +1665,9 @@ public enum O2OneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             PlainString = input.ReadString();
@@ -1847,7 +1867,9 @@ public sealed partial class TestJsonName : pb::IMessage<TestJsonName> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -2047,7 +2069,9 @@ public enum ValueOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Text = input.ReadString();
@@ -2211,7 +2235,9 @@ public sealed partial class Nested : pb::IMessage<Nested> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 X = input.ReadInt32();
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs
index bbbee22c7b6a..1d4367b78151 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs
@@ -1387,7 +1387,9 @@ public enum OneofFieldOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             SingleInt32 = input.ReadInt32();
@@ -1755,7 +1757,9 @@ public sealed partial class NestedMessage : pb::IMessage<NestedMessage> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 Bb = input.ReadInt32();
@@ -1938,7 +1942,9 @@ public sealed partial class NestedTestAllTypes : pb::IMessage<NestedTestAllTypes
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (child_ == null) {
@@ -2082,7 +2088,9 @@ public sealed partial class TestDeprecatedFields : pb::IMessage<TestDeprecatedFi
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             DeprecatedInt32 = input.ReadInt32();
@@ -2215,7 +2223,9 @@ public sealed partial class ForeignMessage : pb::IMessage<ForeignMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             C = input.ReadInt32();
@@ -2320,7 +2330,9 @@ public sealed partial class TestReservedFields : pb::IMessage<TestReservedFields
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -2451,7 +2463,9 @@ public sealed partial class TestForeignNested : pb::IMessage<TestForeignNested>
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (foreignNested_ == null) {
@@ -2614,7 +2628,9 @@ public sealed partial class TestReallyLargeTagNumber : pb::IMessage<TestReallyLa
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             A = input.ReadInt32();
@@ -2774,7 +2790,9 @@ public sealed partial class TestRecursiveMessage : pb::IMessage<TestRecursiveMes
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (a_ == null) {
@@ -2916,7 +2934,9 @@ public sealed partial class TestMutualRecursionA : pb::IMessage<TestMutualRecurs
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (bb_ == null) {
@@ -3075,7 +3095,9 @@ public sealed partial class TestMutualRecursionB : pb::IMessage<TestMutualRecurs
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (a_ == null) {
@@ -3211,7 +3233,9 @@ public sealed partial class TestEnumAllowAlias : pb::IMessage<TestEnumAllowAlias
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Value = (global::Google.Protobuf.TestProtos.TestEnumWithDupValue) input.ReadEnum();
@@ -3483,7 +3507,9 @@ public sealed partial class TestCamelCaseFieldNames : pb::IMessage<TestCamelCase
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             PrimitiveField = input.ReadInt32();
@@ -3724,7 +3750,9 @@ public sealed partial class TestFieldOrderings : pb::IMessage<TestFieldOrderings
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             MyInt = input.ReadInt64();
@@ -3899,7 +3927,9 @@ public sealed partial class NestedMessage : pb::IMessage<NestedMessage> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 Bb = input.ReadInt32();
@@ -4037,7 +4067,9 @@ public sealed partial class SparseEnumMessage : pb::IMessage<SparseEnumMessage>
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             SparseEnum = (global::Google.Protobuf.TestProtos.TestSparseEnum) input.ReadEnum();
@@ -4169,7 +4201,9 @@ public sealed partial class OneString : pb::IMessage<OneString> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Data = input.ReadString();
@@ -4290,7 +4324,9 @@ public sealed partial class MoreString : pb::IMessage<MoreString> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             data_.AddEntriesFrom(input, _repeated_data_codec);
@@ -4419,7 +4455,9 @@ public sealed partial class OneBytes : pb::IMessage<OneBytes> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Data = input.ReadBytes();
@@ -4548,7 +4586,9 @@ public sealed partial class MoreBytes : pb::IMessage<MoreBytes> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Data = input.ReadBytes();
@@ -4680,7 +4720,9 @@ public sealed partial class Int32Message : pb::IMessage<Int32Message> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Data = input.ReadInt32();
@@ -4809,7 +4851,9 @@ public sealed partial class Uint32Message : pb::IMessage<Uint32Message> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Data = input.ReadUInt32();
@@ -4938,7 +4982,9 @@ public sealed partial class Int64Message : pb::IMessage<Int64Message> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Data = input.ReadInt64();
@@ -5067,7 +5113,9 @@ public sealed partial class Uint64Message : pb::IMessage<Uint64Message> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Data = input.ReadUInt64();
@@ -5196,7 +5244,9 @@ public sealed partial class BoolMessage : pb::IMessage<BoolMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Data = input.ReadBool();
@@ -5413,7 +5463,9 @@ public enum FooOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             FooInt = input.ReadInt32();
@@ -5755,7 +5807,9 @@ public sealed partial class TestPackedTypes : pb::IMessage<TestPackedTypes> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 722:
           case 720: {
@@ -6154,7 +6208,9 @@ public sealed partial class TestUnpackedTypes : pb::IMessage<TestUnpackedTypes>
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 722:
           case 720: {
@@ -6435,7 +6491,9 @@ public sealed partial class TestRepeatedScalarDifferentTagSizes : pb::IMessage<T
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 98:
           case 101: {
@@ -6593,7 +6651,9 @@ public sealed partial class TestCommentInjectionMessage : pb::IMessage<TestComme
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             A = input.ReadString();
@@ -6701,7 +6761,9 @@ public sealed partial class FooRequest : pb::IMessage<FooRequest> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -6802,7 +6864,9 @@ public sealed partial class FooResponse : pb::IMessage<FooResponse> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -6903,7 +6967,9 @@ public sealed partial class FooClientMessage : pb::IMessage<FooClientMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -7004,7 +7070,9 @@ public sealed partial class FooServerMessage : pb::IMessage<FooServerMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -7105,7 +7173,9 @@ public sealed partial class BarRequest : pb::IMessage<BarRequest> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -7206,7 +7276,9 @@ public sealed partial class BarResponse : pb::IMessage<BarResponse> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -7307,7 +7379,9 @@ public sealed partial class TestEmptyMessage : pb::IMessage<TestEmptyMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
@@ -7438,7 +7512,9 @@ public sealed partial class CommentMessage : pb::IMessage<CommentMessage> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Text = input.ReadString();
@@ -7585,7 +7661,9 @@ public sealed partial class NestedCommentMessage : pb::IMessage<NestedCommentMes
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 10: {
                 NestedText = input.ReadString();
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs
index 45f8ece64695..a36825dc3297 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs
+++ b/csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs
@@ -788,7 +788,9 @@ public sealed partial class TestWellKnownTypes : pb::IMessage<TestWellKnownTypes
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             if (anyField_ == null) {
@@ -1316,7 +1318,9 @@ public sealed partial class RepeatedWellKnownTypes : pb::IMessage<RepeatedWellKn
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             anyField_.AddEntriesFrom(input, _repeated_anyField_codec);
@@ -2027,7 +2031,9 @@ public enum OneofFieldOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             global::Google.Protobuf.WellKnownTypes.Any subBuilder = new global::Google.Protobuf.WellKnownTypes.Any();
@@ -2538,7 +2544,9 @@ public sealed partial class MapWellKnownTypes : pb::IMessage<MapWellKnownTypes>
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             anyField_.AddEntriesFrom(input, _map_anyField_codec);
diff --git a/csharp/src/Google.Protobuf.Test/testprotos.pb b/csharp/src/Google.Protobuf.Test/testprotos.pb
index 94ff381705ad..69950675d86e 100644
Binary files a/csharp/src/Google.Protobuf.Test/testprotos.pb and b/csharp/src/Google.Protobuf.Test/testprotos.pb differ
diff --git a/csharp/src/Google.Protobuf/CodedInputStream.cs b/csharp/src/Google.Protobuf/CodedInputStream.cs
index 0a829545e257..0428fb54e672 100644
--- a/csharp/src/Google.Protobuf/CodedInputStream.cs
+++ b/csharp/src/Google.Protobuf/CodedInputStream.cs
@@ -373,7 +373,7 @@ public uint ReadTag()
                 if (IsAtEnd)
                 {
                     lastTag = 0;
-                    return 0; // This is the only case in which we return 0.
+                    return 0;
                 }
 
                 lastTag = ReadRawVarint32();
@@ -383,6 +383,10 @@ public uint ReadTag()
                 // If we actually read a tag with a field of 0, that's not a valid tag.
                 throw InvalidProtocolBufferException.InvalidTag();
             }
+            if (ReachedLimit)
+            {
+                return 0;
+            }
             return lastTag;
         }
 
@@ -591,6 +595,20 @@ public void ReadMessage(IMessage builder)
             PopLimit(oldLimit);
         }
 
+        /// <summary>
+        /// Reads an embedded group field from the stream.
+        /// </summary>
+        public void ReadGroup(IMessage builder)
+        {
+            if (recursionDepth >= recursionLimit)
+            {
+                throw InvalidProtocolBufferException.RecursionLimitExceeded();
+            }
+            ++recursionDepth;
+            builder.MergeFrom(this);
+            --recursionDepth;
+        }
+
         /// <summary>
         /// Reads a bytes field value from the stream.
         /// </summary>   
diff --git a/csharp/src/Google.Protobuf/CodedOutputStream.cs b/csharp/src/Google.Protobuf/CodedOutputStream.cs
index 6211aac32cb1..f9ad29086bda 100644
--- a/csharp/src/Google.Protobuf/CodedOutputStream.cs
+++ b/csharp/src/Google.Protobuf/CodedOutputStream.cs
@@ -303,6 +303,15 @@ public void WriteMessage(IMessage value)
             value.WriteTo(this);
         }
 
+        /// <summary>
+        /// Writes a group, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteGroup(IMessage value)
+        {
+            value.WriteTo(this);
+        }
+
         /// <summary>
         /// Write a byte string, without a tag, to the stream.
         /// The data is length-prefixed.
diff --git a/csharp/src/Google.Protobuf/Collections/RepeatedField.cs b/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
index 613ca9c48e7f..827bc711832b 100644
--- a/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
+++ b/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
@@ -208,6 +208,10 @@ public void WriteTo(CodedOutputStream output, FieldCodec<T> codec)
                 {
                     output.WriteTag(tag);
                     writer(output, array[i]);
+                    if (codec.EndTag != 0)
+                    {
+                        output.WriteTag(codec.EndTag);
+                    }
                 }
             }
         }
diff --git a/csharp/src/Google.Protobuf/FieldCodec.cs b/csharp/src/Google.Protobuf/FieldCodec.cs
index a11f2420e313..221ad5fd2d50 100644
--- a/csharp/src/Google.Protobuf/FieldCodec.cs
+++ b/csharp/src/Google.Protobuf/FieldCodec.cs
@@ -225,6 +225,19 @@ public static FieldCodec<T> ForEnum<T>(uint tag, Func<T, int> toInt32, Func<int,
                 (output, value) => output.WriteMessage(value), message => CodedOutputStream.ComputeMessageSize(message), tag);
         }
 
+        /// <summary>
+        /// Retrieves a codec suitable for a group field with the given tag.
+        /// </summary>
+        /// <param name="startTag">The start group tag.</param>
+        /// <param name="endTag">The end group tag.</param>
+        /// <param name="parser">A parser to use for the group message type.</param>
+        /// <returns>A codec for given tag</returns>
+        public static FieldCodec<T> ForGroup<T>(uint startTag, uint endTag, MessageParser<T> parser) where T : IMessage<T>
+        {
+            return new FieldCodec<T>(input => { T message = parser.CreateTemplate(); input.ReadGroup(message); return message; },
+                (output, value) => output.WriteGroup(value), message => CodedOutputStream.ComputeGroupSize(message), startTag, endTag);
+        }
+
         /// <summary>
         /// Creates a codec for a wrapper type of a class - which must be string or ByteString.
         /// </summary>
@@ -235,7 +248,7 @@ public static FieldCodec<T> ForEnum<T>(uint tag, Func<T, int> toInt32, Func<int,
                 input => WrapperCodecs.Read<T>(input, nestedCodec),
                 (output, value) => WrapperCodecs.Write<T>(output, value, nestedCodec),
                 value => WrapperCodecs.CalculateSize<T>(value, nestedCodec),
-                tag,
+                tag, 0,
                 null); // Default value for the wrapper
         }
 
@@ -250,7 +263,7 @@ public static FieldCodec<T> ForEnum<T>(uint tag, Func<T, int> toInt32, Func<int,
                 input => WrapperCodecs.Read<T>(input, nestedCodec),
                 (output, value) => WrapperCodecs.Write<T>(output, value.Value, nestedCodec),
                 value => value == null ? 0 : WrapperCodecs.CalculateSize<T>(value.Value, nestedCodec),
-                tag,
+                tag, 0,
                 null); // Default value for the wrapper
         }
 
@@ -399,6 +412,14 @@ static FieldCodec()
         /// </value>
         internal uint Tag { get; }
 
+        /// <summary>
+        /// Gets the end tag of the codec or 0 if there is no end tag
+        /// </summary>
+        /// <value>
+        /// The end tag of the codec.
+        /// </value>
+        internal uint EndTag { get; }
+
         /// <summary>
         /// Default value for this codec. Usually the same for every instance of the same type, but
         /// for string/ByteString wrapper fields the codec's default value is null, whereas for
@@ -424,7 +445,8 @@ static FieldCodec()
             Func<CodedInputStream, T> reader,
             Action<CodedOutputStream, T> writer,
             Func<T, int> sizeCalculator,
-            uint tag) : this(reader, writer, sizeCalculator, tag, DefaultDefault)
+            uint tag,
+            uint endTag = 0) : this(reader, writer, sizeCalculator, tag, endTag, DefaultDefault)
         {
         }
 
@@ -433,6 +455,7 @@ static FieldCodec()
             Action<CodedOutputStream, T> writer,
             Func<T, int> sizeCalculator,
             uint tag,
+            uint endTag,
             T defaultValue)
         {
             ValueReader = reader;
@@ -455,6 +478,10 @@ public void WriteTagAndValue(CodedOutputStream output, T value)
             {
                 output.WriteTag(Tag);
                 ValueWriter(output, value);
+                if (EndTag != 0)
+                {
+                    output.WriteTag(EndTag);
+                }
             }
         }
 
diff --git a/csharp/src/Google.Protobuf/Reflection/Descriptor.cs b/csharp/src/Google.Protobuf/Reflection/Descriptor.cs
index 0c33e63df46a..f101cd35265c 100644
--- a/csharp/src/Google.Protobuf/Reflection/Descriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/Descriptor.cs
@@ -302,7 +302,9 @@ internal sealed partial class FileDescriptorSet : pb::IMessage<FileDescriptorSet
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             file_.AddEntriesFrom(input, _repeated_file_codec);
@@ -733,7 +735,9 @@ internal sealed partial class FileDescriptorProto : pb::IMessage<FileDescriptorP
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -1098,7 +1102,9 @@ internal sealed partial class DescriptorProto : pb::IMessage<DescriptorProto> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -1357,7 +1363,9 @@ internal sealed partial class ExtensionRange : pb::IMessage<ExtensionRange> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 Start = input.ReadInt32();
@@ -1560,7 +1568,9 @@ internal sealed partial class ReservedRange : pb::IMessage<ReservedRange> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 Start = input.ReadInt32();
@@ -1693,7 +1703,9 @@ internal sealed partial class ExtensionRangeOptions : pb::IMessage<ExtensionRang
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 7994: {
             uninterpretedOption_.AddEntriesFrom(input, _repeated_uninterpretedOption_codec);
@@ -2200,7 +2212,9 @@ internal sealed partial class FieldDescriptorProto : pb::IMessage<FieldDescripto
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -2487,7 +2501,9 @@ internal sealed partial class OneofDescriptorProto : pb::IMessage<OneofDescripto
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -2732,7 +2748,9 @@ internal sealed partial class EnumDescriptorProto : pb::IMessage<EnumDescriptorP
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -2948,7 +2966,9 @@ internal sealed partial class EnumReservedRange : pb::IMessage<EnumReservedRange
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 8: {
                 Start = input.ReadInt32();
@@ -3177,7 +3197,9 @@ internal sealed partial class EnumValueDescriptorProto : pb::IMessage<EnumValueD
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -3385,7 +3407,9 @@ internal sealed partial class ServiceDescriptorProto : pb::IMessage<ServiceDescr
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -3735,7 +3759,9 @@ internal sealed partial class MethodDescriptorProto : pb::IMessage<MethodDescrip
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -6923,7 +6949,9 @@ internal sealed partial class UninterpretedOption : pb::IMessage<UninterpretedOp
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 18: {
             name_.AddEntriesFrom(input, _repeated_name_codec);
@@ -7136,7 +7164,9 @@ internal sealed partial class NamePart : pb::IMessage<NamePart> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 10: {
                 NamePart_ = input.ReadString();
@@ -7315,7 +7345,9 @@ internal sealed partial class SourceCodeInfo : pb::IMessage<SourceCodeInfo> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             location_.AddEntriesFrom(input, _repeated_location_codec);
@@ -7623,7 +7655,9 @@ internal sealed partial class Location : pb::IMessage<Location> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 10:
               case 8: {
@@ -7776,7 +7810,9 @@ internal sealed partial class GeneratedCodeInfo : pb::IMessage<GeneratedCodeInfo
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             annotation_.AddEntriesFrom(input, _repeated_annotation_codec);
@@ -8027,7 +8063,9 @@ internal sealed partial class Annotation : pb::IMessage<Annotation> {
           while ((tag = input.ReadTag()) != 0) {
             switch(tag) {
               default:
-                _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+                if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+                  return;
+                }
                 break;
               case 10:
               case 8: {
diff --git a/csharp/src/Google.Protobuf/UnknownField.cs b/csharp/src/Google.Protobuf/UnknownField.cs
index 0d6eed6357a8..2b13fdb3ec6a 100644
--- a/csharp/src/Google.Protobuf/UnknownField.cs
+++ b/csharp/src/Google.Protobuf/UnknownField.cs
@@ -55,6 +55,7 @@ internal sealed class UnknownField
         private List<uint> fixed32List;
         private List<ulong> fixed64List;
         private List<ByteString> lengthDelimitedList;
+        private List<UnknownFieldSet> groupList;
 
         /// <summary>
         /// Creates a new UnknownField.
@@ -77,7 +78,8 @@ public override bool Equals(object other)
                    && Lists.Equals(varintList, otherField.varintList)
                    && Lists.Equals(fixed32List, otherField.fixed32List)
                    && Lists.Equals(fixed64List, otherField.fixed64List)
-                   && Lists.Equals(lengthDelimitedList, otherField.lengthDelimitedList);
+                   && Lists.Equals(lengthDelimitedList, otherField.lengthDelimitedList)
+                   && Lists.Equals(groupList, otherField.groupList);
         }
 
         /// <summary>
@@ -90,6 +92,7 @@ public override int GetHashCode()
             hash = hash * 47 + Lists.GetHashCode(fixed32List);
             hash = hash * 47 + Lists.GetHashCode(fixed64List);
             hash = hash * 47 + Lists.GetHashCode(lengthDelimitedList);
+            hash = hash * 47 + Lists.GetHashCode(groupList);
             return hash;
         }
 
@@ -133,6 +136,15 @@ internal void WriteTo(int fieldNumber, CodedOutputStream output)
                     output.WriteBytes(value);
                 }
             }
+            if (groupList != null)
+            {
+                foreach (UnknownFieldSet value in groupList)
+                {
+                    output.WriteTag(fieldNumber, WireFormat.WireType.StartGroup);
+                    value.WriteTo(output);
+                    output.WriteTag(fieldNumber, WireFormat.WireType.EndGroup);
+                }
+            }
         }
 
         /// <summary>
@@ -168,6 +180,14 @@ internal int GetSerializedSize(int fieldNumber)
                     result += CodedOutputStream.ComputeBytesSize(value);
                 }
             }
+            if (groupList != null)
+            {
+                result += CodedOutputStream.ComputeTagSize(fieldNumber) * 2 * groupList.Count;
+                foreach (UnknownFieldSet value in groupList)
+                {
+                    result += value.CalculateSize();
+                }
+            }
             return result;
         }
 
@@ -182,6 +202,7 @@ internal UnknownField MergeFrom(UnknownField other)
             fixed32List = AddAll(fixed32List, other.fixed32List);
             fixed64List = AddAll(fixed64List, other.fixed64List);
             lengthDelimitedList = AddAll(lengthDelimitedList, other.lengthDelimitedList);
+            groupList = AddAll(groupList, other.groupList);
             return this;
         }
 
@@ -245,6 +266,12 @@ internal UnknownField AddLengthDelimited(ByteString value)
             return this;
         }
 
+        internal UnknownField AddGroup(UnknownFieldSet value) 
+        {
+            groupList = Add(groupList, value);
+            return this;
+        }
+
         /// <summary>
         /// Adds <paramref name="value"/> to the <paramref name="list"/>, creating
         /// a new list if <paramref name="list"/> is null. The list is returned - either
diff --git a/csharp/src/Google.Protobuf/UnknownFieldSet.cs b/csharp/src/Google.Protobuf/UnknownFieldSet.cs
index 6404c3c08f85..9121567621a1 100644
--- a/csharp/src/Google.Protobuf/UnknownFieldSet.cs
+++ b/csharp/src/Google.Protobuf/UnknownFieldSet.cs
@@ -183,7 +183,7 @@ internal UnknownFieldSet AddOrReplaceField(int number, UnknownField field)
         /// </summary>
         /// <param name="input">The coded input stream containing the field</param>
         /// <returns>false if the tag is an "end group" tag, true otherwise</returns>
-        private void MergeFieldFrom(CodedInputStream input)
+        private bool MergeFieldFrom(CodedInputStream input)
         {
             uint tag = input.LastTag;
             int number = WireFormat.GetTagFieldNumber(tag);
@@ -193,34 +193,40 @@ private void MergeFieldFrom(CodedInputStream input)
                     {
                         ulong uint64 = input.ReadUInt64();
                         GetOrAddField(number).AddVarint(uint64);
-                        return;
+                        return true;
                     }
                 case WireFormat.WireType.Fixed32:
                     {
                         uint uint32 = input.ReadFixed32();
                         GetOrAddField(number).AddFixed32(uint32);
-                        return;
+                        return true;
                     }
                 case WireFormat.WireType.Fixed64:
                     {
                         ulong uint64 = input.ReadFixed64();
                         GetOrAddField(number).AddFixed64(uint64);
-                        return;
+                        return true;
                     }
                 case WireFormat.WireType.LengthDelimited:
                     {
                         ByteString bytes = input.ReadBytes();
                         GetOrAddField(number).AddLengthDelimited(bytes);
-                        return;
+                        return true;
                     }
                 case WireFormat.WireType.StartGroup:
                     {
-                        input.SkipGroup(tag);
-                        return;
+                        uint endTag = WireFormat.MakeTag(number, WireFormat.WireType.EndGroup);
+                        UnknownFieldSet set = new UnknownFieldSet();
+                        while (input.ReadTag() != endTag)
+                        {
+                            set.MergeFieldFrom(input);
+                        }
+                        GetOrAddField(number).AddGroup(set);
+                        return true;
                     }
                 case WireFormat.WireType.EndGroup:
                     {
-                        throw new InvalidProtocolBufferException("Merge an unknown field of end-group tag, indicating that the corresponding start-group was missing.");
+                        return false;
                     }
                 default:
                     throw new InvalidOperationException("Wire Type is invalid.");
@@ -248,8 +254,34 @@ private void MergeFieldFrom(CodedInputStream input)
             {
                 unknownFields = new UnknownFieldSet();
             }
-            unknownFields.MergeFieldFrom(input);
+            if (!unknownFields.MergeFieldFrom(input))
+            {
+                throw new InvalidProtocolBufferException("Merge an unknown field of end-group tag, indicating that the corresponding start-group was missing."); // match the old code-gen
+            }
             return unknownFields;
+        }
+
+        /// <summary>
+        /// Create a new UnknownFieldSet if unknownFields is null.
+        /// Parse a single field from <paramref name="input"/> and merge it
+        /// into unknownFields. If <paramref name="input"/> is configured to discard unknown fields,
+        /// <paramref name="unknownFields"/> will be returned as-is and the field will be skipped.
+        /// </summary>
+        /// <param name="unknownFields">The UnknownFieldSet which need to be merged</param>
+        /// <param name="input">The coded input stream containing the field</param>
+        /// <returns>The merged UnknownFieldSet</returns>
+        public static bool MergeFieldFrom(ref UnknownFieldSet unknownFields, CodedInputStream input)
+        {
+            if (input.DiscardUnknownFields)
+            {
+                input.SkipLastField();
+                return true;
+            }
+            if (unknownFields == null)
+            {
+                unknownFields = new UnknownFieldSet();
+            }
+            return unknownFields.MergeFieldFrom(input);
         }
 
         /// <summary>
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Any.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Any.cs
index dd9911067ed9..b1ed0bc3077c 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Any.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Any.cs
@@ -292,7 +292,9 @@ public sealed partial class Any : pb::IMessage<Any> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             TypeUrl = input.ReadString();
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Api.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Api.cs
index 438e1db8da2f..a566e406403b 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Api.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Api.cs
@@ -345,7 +345,9 @@ public sealed partial class Api : pb::IMessage<Api> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -661,7 +663,9 @@ public sealed partial class Method : pb::IMessage<Method> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -925,7 +929,9 @@ public sealed partial class Mixin : pb::IMessage<Mixin> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Duration.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Duration.cs
index 2858b532b636..691ca334ea2b 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Duration.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Duration.cs
@@ -254,7 +254,9 @@ public sealed partial class Duration : pb::IMessage<Duration> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Seconds = input.ReadInt64();
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Empty.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Empty.cs
index 2113add9bca1..03ffc4a05776 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Empty.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Empty.cs
@@ -143,7 +143,9 @@ public sealed partial class Empty : pb::IMessage<Empty> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
         }
       }
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/FieldMask.cs b/csharp/src/Google.Protobuf/WellKnownTypes/FieldMask.cs
index 6ad31a50eacc..8114aa3924ef 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/FieldMask.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/FieldMask.cs
@@ -352,7 +352,9 @@ public sealed partial class FieldMask : pb::IMessage<FieldMask> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             paths_.AddEntriesFrom(input, _repeated_paths_codec);
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/SourceContext.cs b/csharp/src/Google.Protobuf/WellKnownTypes/SourceContext.cs
index 124ddaa71234..2cae68631df8 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/SourceContext.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/SourceContext.cs
@@ -165,7 +165,9 @@ public sealed partial class SourceContext : pb::IMessage<SourceContext> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             FileName = input.ReadString();
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Struct.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Struct.cs
index 194b81e965e5..9667472522e2 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Struct.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Struct.cs
@@ -189,7 +189,9 @@ public sealed partial class Struct : pb::IMessage<Struct> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             fields_.AddEntriesFrom(input, _map_fields_codec);
@@ -513,7 +515,9 @@ public enum KindOneofCase {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             kind_ = input.ReadEnum();
@@ -673,7 +677,9 @@ public sealed partial class ListValue : pb::IMessage<ListValue> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             values_.AddEntriesFrom(input, _repeated_values_codec);
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Timestamp.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Timestamp.cs
index 984e226fd449..9e37e9dda7de 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Timestamp.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Timestamp.cs
@@ -273,7 +273,9 @@ public sealed partial class Timestamp : pb::IMessage<Timestamp> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Seconds = input.ReadInt64();
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Type.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Type.cs
index 52bd343ba841..adea91063496 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Type.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Type.cs
@@ -328,7 +328,9 @@ public sealed partial class Type : pb::IMessage<Type> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -723,7 +725,9 @@ public sealed partial class Field : pb::IMessage<Field> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Kind = (global::Google.Protobuf.WellKnownTypes.Field.Types.Kind) input.ReadEnum();
@@ -1100,7 +1104,9 @@ public sealed partial class Enum : pb::IMessage<Enum> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -1300,7 +1306,9 @@ public sealed partial class EnumValue : pb::IMessage<EnumValue> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
@@ -1480,7 +1488,9 @@ public sealed partial class Option : pb::IMessage<Option> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Name = input.ReadString();
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/Wrappers.cs b/csharp/src/Google.Protobuf/WellKnownTypes/Wrappers.cs
index 25a65aa72c98..679eb0f23442 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/Wrappers.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/Wrappers.cs
@@ -177,7 +177,9 @@ public sealed partial class DoubleValue : pb::IMessage<DoubleValue> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 9: {
             Value = input.ReadDouble();
@@ -314,7 +316,9 @@ public sealed partial class FloatValue : pb::IMessage<FloatValue> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 13: {
             Value = input.ReadFloat();
@@ -451,7 +455,9 @@ public sealed partial class Int64Value : pb::IMessage<Int64Value> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Value = input.ReadInt64();
@@ -588,7 +594,9 @@ public sealed partial class UInt64Value : pb::IMessage<UInt64Value> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Value = input.ReadUInt64();
@@ -725,7 +733,9 @@ public sealed partial class Int32Value : pb::IMessage<Int32Value> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Value = input.ReadInt32();
@@ -862,7 +872,9 @@ public sealed partial class UInt32Value : pb::IMessage<UInt32Value> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Value = input.ReadUInt32();
@@ -999,7 +1011,9 @@ public sealed partial class BoolValue : pb::IMessage<BoolValue> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 8: {
             Value = input.ReadBool();
@@ -1136,7 +1150,9 @@ public sealed partial class StringValue : pb::IMessage<StringValue> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Value = input.ReadString();
@@ -1273,7 +1289,9 @@ public sealed partial class BytesValue : pb::IMessage<BytesValue> {
       while ((tag = input.ReadTag()) != 0) {
         switch(tag) {
           default:
-            _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);
+            if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {
+              return;
+            }
             break;
           case 10: {
             Value = input.ReadBytes();
diff --git a/src/google/protobuf/compiler/csharp/csharp_message.cc b/src/google/protobuf/compiler/csharp/csharp_message.cc
index 124e673b3524..be1cd115a52b 100644
--- a/src/google/protobuf/compiler/csharp/csharp_message.cc
+++ b/src/google/protobuf/compiler/csharp/csharp_message.cc
@@ -539,7 +539,9 @@ void MessageGenerator::GenerateMergingMethods(io::Printer* printer) {
   } else {
     printer->Print(
       "default:\n"
-      "  _unknownFields = pb::UnknownFieldSet.MergeFieldFrom(_unknownFields, input);\n"
+      "  if (!pb::UnknownFieldSet.MergeFieldFrom(ref _unknownFields, input)) {\n"
+      "    return;\n"
+      "  }\n"
       "  break;\n");
   }
   for (int i = 0; i < fields_by_number().size(); i++) {
diff --git a/src/google/protobuf/compiler/csharp/csharp_message_field.cc b/src/google/protobuf/compiler/csharp/csharp_message_field.cc
index 16714603671f..899cd7bf7bd9 100644
--- a/src/google/protobuf/compiler/csharp/csharp_message_field.cc
+++ b/src/google/protobuf/compiler/csharp/csharp_message_field.cc
@@ -37,6 +37,8 @@
 #include <google/protobuf/io/printer.h>
 #include <google/protobuf/io/zero_copy_stream.h>
 #include <google/protobuf/stubs/strutil.h>
+#include <google/protobuf/wire_format.h>
+#include <google/protobuf/wire_format_lite.h>
 
 #include <google/protobuf/compiler/csharp/csharp_doc_comment.h>
 #include <google/protobuf/compiler/csharp/csharp_helpers.h>
@@ -56,6 +58,21 @@ MessageFieldGenerator::MessageFieldGenerator(const FieldDescriptor* descriptor,
     variables_["has_property_check"] = name() + "_ != null";
     variables_["has_not_property_check"] = name() + "_ == null";
   }
+
+  if (descriptor_->type() == FieldDescriptor::Type::TYPE_GROUP) {
+    int tag_size = internal::WireFormat::TagSize(descriptor_->number(), descriptor_->type()) / 2;
+    uint tag = internal::WireFormatLite::MakeTag(
+        descriptor_->number(),
+        internal::WireFormatLite::WIRETYPE_END_GROUP);
+    uint8 tag_array[5];
+    io::CodedOutputStream::WriteTagToArray(tag, tag_array);
+    string tag_bytes = SimpleItoa(tag_array[0]);
+    for (int i = 1; i < tag_size; i++) {
+        tag_bytes += ", " + SimpleItoa(tag_array[i]);
+    }
+    variables_["end_tag"] = SimpleItoa(tag);
+    variables_["end_tag_bytes"] = tag_bytes;
+  }
 }
 
 MessageFieldGenerator::~MessageFieldGenerator() {
@@ -114,25 +131,47 @@ void MessageFieldGenerator::GenerateParsingCode(io::Printer* printer) {
     variables_,
     "if ($has_not_property_check$) {\n"
     "  $property_name$ = new $type_name$();\n"
-    "}\n"
-    "input.ReadMessage($property_name$);\n");
+    "}\n");
+  if (descriptor_->type() == FieldDescriptor::Type::TYPE_MESSAGE) {
+    printer->Print(variables_, "input.ReadMessage($property_name$);\n");
+  } else {
+    printer->Print(variables_, "input.ReadGroup($property_name$);\n");
+  }
 }
 
 void MessageFieldGenerator::GenerateSerializationCode(io::Printer* printer) {
-  printer->Print(
-    variables_,
-    "if ($has_property_check$) {\n"
-    "  output.WriteRawTag($tag_bytes$);\n"
-    "  output.WriteMessage($property_name$);\n"
-    "}\n");
+  if (descriptor_->type() == FieldDescriptor::Type::TYPE_MESSAGE) {
+    printer->Print(
+      variables_,
+      "if ($has_property_check$) {\n"
+      "  output.WriteRawTag($tag_bytes$);\n"
+      "  output.WriteMessage($property_name$);\n"
+      "}\n");
+  } else {
+    printer->Print(
+      variables_,
+      "if ($has_property_check$) {\n"
+      "  output.WriteRawTag($tag_bytes$);\n"
+      "  output.WriteGroup($property_name$);\n"
+      "  output.WriteRawTag($end_tag_bytes$);\n"
+      "}\n");
+  }
 }
 
 void MessageFieldGenerator::GenerateSerializedSizeCode(io::Printer* printer) {
-  printer->Print(
-    variables_,
-    "if ($has_property_check$) {\n"
-    "  size += $tag_size$ + pb::CodedOutputStream.ComputeMessageSize($property_name$);\n"
-    "}\n");
+  if (descriptor_->type() == FieldDescriptor::Type::TYPE_MESSAGE) {
+    printer->Print(
+      variables_,
+      "if ($has_property_check$) {\n"
+      "  size += $tag_size$ + pb::CodedOutputStream.ComputeMessageSize($property_name$);\n"
+      "}\n");
+  } else {
+    printer->Print(
+      variables_,
+      "if ($has_property_check$) {\n"
+      "  size += $tag_size$ + $tag_size$ + pb::CodedOutputStream.ComputeGroupSize($property_name$);\n"
+      "}\n");
+  }
 }
 
 void MessageFieldGenerator::WriteHash(io::Printer* printer) {
@@ -160,9 +199,15 @@ void MessageFieldGenerator::GenerateFreezingCode(io::Printer* printer) {
 }
 
 void MessageFieldGenerator::GenerateCodecCode(io::Printer* printer) {
-  printer->Print(
-    variables_,
-    "pb::FieldCodec.ForMessage($tag$, $type_name$.Parser)");
+  if (descriptor_->type() == FieldDescriptor::Type::TYPE_MESSAGE) {
+    printer->Print(
+      variables_,
+      "pb::FieldCodec.ForMessage($tag$, $type_name$.Parser)");
+  } else {
+    printer->Print(
+      variables_,
+      "pb::FieldCodec.ForGroup($tag$, $end_tag$, $type_name$.Parser)");
+  }
 }
 
 MessageOneofFieldGenerator::MessageOneofFieldGenerator(
@@ -228,9 +273,13 @@ void MessageOneofFieldGenerator::GenerateParsingCode(io::Printer* printer) {
     "$type_name$ subBuilder = new $type_name$();\n"
     "if ($has_property_check$) {\n"
     "  subBuilder.MergeFrom($property_name$);\n"
-    "}\n"
-    "input.ReadMessage(subBuilder);\n"
-    "$property_name$ = subBuilder;\n");
+    "}\n");
+  if (descriptor_->type() == FieldDescriptor::Type::TYPE_MESSAGE) {
+    printer->Print("input.ReadMessage(subBuilder);\n");
+  } else {
+    printer->Print("input.ReadGroup(subBuilder);\n");
+  }
+  printer->Print(variables_, "$property_name$ = subBuilder;\n");
 }
 
 void MessageOneofFieldGenerator::WriteToString(io::Printer* printer) {