diff --git a/csharp/src/Google.Protobuf.Test/JsonParserTest.cs b/csharp/src/Google.Protobuf.Test/JsonParserTest.cs
index a030aa685f62..87a389aecbb5 100644
--- a/csharp/src/Google.Protobuf.Test/JsonParserTest.cs
+++ b/csharp/src/Google.Protobuf.Test/JsonParserTest.cs
@@ -551,7 +551,7 @@ public void NumberToDouble_Valid(string jsonValue, double expectedParsedValue)
         }
 
         [Test]
-#if !NET50
+#if !NET5_0
         [TestCase("1.7977e308")]
         [TestCase("-1.7977e308")]
         [TestCase("1e309")]
diff --git a/csharp/src/Google.Protobuf.Test/JsonTokenizerTest.cs b/csharp/src/Google.Protobuf.Test/JsonTokenizerTest.cs
index 2de0304b29cc..55ec02ea023f 100644
--- a/csharp/src/Google.Protobuf.Test/JsonTokenizerTest.cs
+++ b/csharp/src/Google.Protobuf.Test/JsonTokenizerTest.cs
@@ -199,7 +199,7 @@ public void NumberValue(string json, double expectedValue)
         [TestCase("1e-")]
         [TestCase("--")]
         [TestCase("--1")]
-#if !NET50
+#if !NET5_0
         [TestCase("-1.7977e308")]
         [TestCase("1.7977e308")]
 #endif
diff --git a/csharp/src/Google.Protobuf/WritingPrimitives.cs b/csharp/src/Google.Protobuf/WritingPrimitives.cs
index d4c2dcef9c35..d8e1fbe7dd80 100644
--- a/csharp/src/Google.Protobuf/WritingPrimitives.cs
+++ b/csharp/src/Google.Protobuf/WritingPrimitives.cs
@@ -53,7 +53,7 @@ internal static class WritingPrimitives
 #if NET5_0
         internal static Encoding Utf8Encoding => Encoding.UTF8; // allows JIT to devirtualize
 #else
-        internal static Encoding Utf8Encoding = Encoding.UTF8; // "Local" copy of Encoding.UTF8, for efficiency. (Yes, it makes a difference.)
+        internal static readonly Encoding Utf8Encoding = Encoding.UTF8; // "Local" copy of Encoding.UTF8, for efficiency. (Yes, it makes a difference.)
 #endif
 
         #region Writing of values (not including tags)
@@ -179,6 +179,8 @@ public static void WriteString(ref Span<byte> buffer, ref WriterInternalState st
             {
                 if (length == value.Length) // Must be all ASCII...
                 {
+                    int currentIndex = 0;
+
                     // If 64bit, and value has at least 4 chars, process 4 chars at a time.
                     if (IntPtr.Size == 8 && value.Length >= 4)
                     {
@@ -187,29 +189,23 @@ public static void WriteString(ref Span<byte> buffer, ref WriterInternalState st
 
                         // Process 4 chars at a time until there are less than 4 remaining.
                         // We already know all characters are ASCII so there is no need to validate the source.
-                        int currentIndex = 0;
-                        int lastIndexWhereCanReadFourChars = (2 * value.Length - 8);
+                        int lastIndexWhereCanReadFourChars = value.Length - 4;
                         do
                         {
                             NarrowFourUtf16CharsToAsciiAndWriteToBuffer(
-                                ref Unsafe.AddByteOffset(ref destinationBytes, (IntPtr)(currentIndex / 2)),
-                                Unsafe.ReadUnaligned<ulong>(ref Unsafe.AddByteOffset(ref sourceBytes, (IntPtr)currentIndex)));
+                                ref Unsafe.AddByteOffset(ref destinationBytes, (IntPtr)currentIndex),
+                                Unsafe.ReadUnaligned<ulong>(ref Unsafe.AddByteOffset(ref sourceBytes, (IntPtr)(currentIndex * 2))));
 
-                        } while ((currentIndex += 8) <= lastIndexWhereCanReadFourChars);
+                        } while ((currentIndex += 4) <= lastIndexWhereCanReadFourChars);
 
-                        // Process remainder.
-                        for (int i = currentIndex / 2; i < length; i++)
-                        {
-                            buffer[state.position + i] = (byte)value[i];
-                        }
+                        // Any chars remaining are processed in for loop below.
                     }
-                    else
+
+                    for (int i = currentIndex; i < length; i++)
                     {
-                        for (int i = 0; i < length; i++)
-                        {
-                            buffer[state.position + i] = (byte)value[i];
-                        }
+                        buffer[state.position + i] = (byte)value[i];
                     }
+
                     state.position += length;
                 }
                 else