${when}, support for RawValue on Inner and Else (backport)

src/NLog/LayoutRenderers/Wrappers/WhenLayoutRendererWrapper.cs

@@ -38,6 +38,7 @@ namespace NLog.LayoutRenderers.Wrappers
     using NLog.Conditions;
     using NLog.Config;
     using NLog.Layouts;
+    using NLog.Internal;
 
     /// <summary>
     /// Only outputs the inner layout when the specified condition has been met.
@@ -46,7 +47,7 @@ namespace NLog.LayoutRenderers.Wrappers
     [AmbientProperty("When")]
     [ThreadAgnostic]
     [ThreadSafe]
-    public sealed class WhenLayoutRendererWrapper : WrapperLayoutRendererBuilderBase
+    public sealed class WhenLayoutRendererWrapper : WrapperLayoutRendererBuilderBase, IRawValue
     {
         /// <summary>
         /// Gets or sets the condition that must be met for the <see cref="WrapperLayoutRendererBase.Inner"/> layout to be printed.
@@ -67,13 +68,13 @@ protected override void Append(StringBuilder builder, LogEventInfo logEvent)
             int orgLength = builder.Length;
             try
             {
-                if (When == null || true.Equals(When.Evaluate(logEvent)))
+                if (ShouldRenderInner(logEvent))
                 {
                     Inner?.RenderAppendBuilder(logEvent, builder);
                 }
-                else if (Else != null)
+                else
                 {
-                    Else.RenderAppendBuilder(logEvent, builder);
+                    Else?.RenderAppendBuilder(logEvent, builder);
                 }
             }
             catch
@@ -83,10 +84,37 @@ protected override void Append(StringBuilder builder, LogEventInfo logEvent)
             }
         }
 
+        private bool ShouldRenderInner(LogEventInfo logEvent)
+        {
+            return When == null || true.Equals(When.Evaluate(logEvent));
+        }
+
         /// <inheritdoc/>
         [Obsolete("Inherit from WrapperLayoutRendererBase and override RenderInnerAndTransform() instead. Marked obsolete in NLog 4.6")]
         protected override void TransformFormattedMesssage(StringBuilder target)
         {
         }
+
+        /// <inheritdoc />
+        public bool TryGetRawValue(LogEventInfo logEvent, out object value)
+        {
+            if (ShouldRenderInner(logEvent))
+            {
+                return TryGetRawValueFromLayout(logEvent, Inner, out value);
+            }
+
+            return TryGetRawValueFromLayout(logEvent, Else, out value);
+        }
+
+        private static bool TryGetRawValueFromLayout(LogEventInfo logEvent, Layout layout, out object value)
+        {
+            if (layout == null)
+            {
+                value = null;
+                return false;
+            }
+
+            return layout.TryGetRawValue(logEvent, out value);
+        }
     }
 }

tests/NLog.UnitTests/LayoutRenderers/Wrappers/WhenTests.cs

@@ -31,6 +31,7 @@
 // THE POSSIBILITY OF SUCH DAMAGE.
 // 
 
+using System;
 using NLog.Config;
 
 namespace NLog.UnitTests.LayoutRenderers.Wrappers
@@ -203,5 +204,49 @@ private static void WhenNumericAndPropertyConditionTest_inner(SimpleLayout l, ob
             le.Properties["Elapsed"] = time;
             Assert.Equal(fast ? "Fast" : "Slow", l.Render(le));
         }
+
+        [Theory]
+        [InlineData("logger", "DBNullValue", true)]
+        [InlineData("logger1", null, false)]
+        public void WhenDbNullRawValueShouldWork(string loggername, object expectedValue, bool expectedSuccess)
+        {
+            expectedValue = OptionalConvert(expectedValue);
+
+            //else cannot be invoked ambiently. First param is inner
+            SimpleLayout l = @"${when:${db-null}:when=logger=='logger':else=better}";
+
+            var le = LogEventInfo.Create(LogLevel.Info, loggername, "message");
+            var success = l.TryGetRawValue(le, out var result);
+
+            Assert.Equal(expectedValue, result);
+            Assert.Equal(expectedSuccess, success);
+        }       
+        [Theory]
+        [InlineData("logger1", "DBNullValue", true)]
+        [InlineData("logger", null, false)]
+        public void WhenDbNullRawValueShouldWorkElse(string loggername, object expectedValue, bool expectedSuccess)
+        {
+
+            expectedValue = OptionalConvert(expectedValue);
+
+            //else cannot be invoked ambiently. First param is inner
+            SimpleLayout l = @"${when:something:when=logger=='logger':else=${db-null}}";
+
+            var le = LogEventInfo.Create(LogLevel.Info, loggername, "message");
+            var success = l.TryGetRawValue(le, out var result);
+
+            Assert.Equal(expectedValue, result);
+            Assert.Equal(expectedSuccess, success);
+        }
+
+        private static object OptionalConvert(object expectedValue)
+        {
+            if (expectedValue is string s && s == "DBNullValue")
+            {
+                expectedValue = DBNull.Value;
+            }
+
+            return expectedValue;
+        }
     }
 }