${whenEmpty} implemented IRawValue and IStringValueRenderer

src/NLog/LayoutRenderers/Wrappers/WhenEmptyLayoutRendererWrapper.cs

@@ -36,17 +36,17 @@ namespace NLog.LayoutRenderers.Wrappers
     using System;
     using System.Text;
     using NLog.Config;
+    using NLog.Internal;
     using NLog.Layouts;
 
     /// <summary>
     /// Outputs alternative layout when the inner layout produces empty result.
     /// </summary>
     [LayoutRenderer("whenEmpty")]
     [AmbientProperty("WhenEmpty")]
-    [AppDomainFixedOutput]
     [ThreadAgnostic]
     [ThreadSafe]
-    public sealed class WhenEmptyLayoutRendererWrapper : WrapperLayoutRendererBuilderBase
+    public sealed class WhenEmptyLayoutRendererWrapper : WrapperLayoutRendererBuilderBase, IRawValue, IStringValueRenderer
     {
         /// <summary>
         /// Gets or sets the layout to be rendered when original layout produced empty result.
@@ -55,11 +55,14 @@ public sealed class WhenEmptyLayoutRendererWrapper : WrapperLayoutRendererBuilde
         [RequiredParameter]
         public Layout WhenEmpty { get; set; }
 
+        private bool _skipStringValueRenderer;
+
         /// <inheritdoc/>
         protected override void InitializeLayoutRenderer()
         {
             base.InitializeLayoutRenderer();
             WhenEmpty?.Initialize(LoggingConfiguration);
+            _skipStringValueRenderer = !TryGetStringValue(out var _, out var _);
         }
 
         /// <inheritdoc/>
@@ -73,6 +76,65 @@ protected override void RenderInnerAndTransform(LogEventInfo logEvent, StringBui
             WhenEmpty.RenderAppendBuilder(logEvent, builder);
         }
 
+        string IStringValueRenderer.GetFormattedString(LogEventInfo logEvent)
+        {
+            if (!_skipStringValueRenderer)
+            {
+                if (TryGetStringValue(out var innerLayout, out var whenEmptyLayout))
+                {
+                    var innerValue = innerLayout.Render(logEvent);
+                    if (!string.IsNullOrEmpty(innerValue))
+                    {
+                        return innerValue;
+                    }
+
+                    // render WhenEmpty when the inner layout was empty
+                    return whenEmptyLayout.Render(logEvent);
+                }
+
+                _skipStringValueRenderer = true;
+            }
+            return null;
+        }
+
+        bool TryGetStringValue(out SimpleLayout innerLayout, out SimpleLayout whenEmptyLayout)
+        {
+            innerLayout = Inner as SimpleLayout;
+            whenEmptyLayout = WhenEmpty as SimpleLayout;
+
+            if (innerLayout == null || (!innerLayout.IsFixedText && !innerLayout.IsSimpleStringText))
+                return false;
+
+            if (whenEmptyLayout == null || (!whenEmptyLayout.IsFixedText && !whenEmptyLayout.IsSimpleStringText))
+                return false;
+
+            return true;
+        }
+
+        bool IRawValue.TryGetRawValue(LogEventInfo logEvent, out object value)
+        {
+            if (Inner.TryGetRawValue(logEvent, out var innerValue))
+            {
+                if (innerValue != null && !innerValue.Equals(string.Empty))
+                {
+                    value = innerValue;
+                    return true;
+                }
+            }
+            else
+            {
+                var innerResult = Inner.Render(logEvent); // Beware this can be very expensive call!
+                if (!string.IsNullOrEmpty(innerResult))
+                {
+                    value = null;
+                    return false;
+                }
+            }
+
+            // render WhenEmpty when the inner layout was empty
+            return WhenEmpty.TryGetRawValue(logEvent, out value);
+        }
+
         /// <inheritdoc/>
         [Obsolete("Inherit from WrapperLayoutRendererBase and override RenderInnerAndTransform() instead. Marked obsolete in NLog 4.6")]
         protected override void TransformFormattedMesssage(StringBuilder target)

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

@@ -33,6 +33,7 @@
 
 namespace NLog.UnitTests.LayoutRenderers.Wrappers
 {
+    using System;
     using NLog;
     using NLog.Layouts;
     using Xunit;
@@ -73,5 +74,26 @@ public void WhenEmpty_MissingInner_ShouldNotThrow()
             var le = LogEventInfo.Create(LogLevel.Info, "logger", "message");
             Assert.Equal("api.log", l.Render(le));
         }
+
+        [Fact]
+        public void WhenDbNullRawValueShouldWork()
+        {
+            SimpleLayout l = @"${event-properties:prop1:whenEmpty=${db-null}}";
+            {
+                var le = LogEventInfo.Create(LogLevel.Info, "logger", "message");
+                le.Properties["prop1"] = 1;
+                var success = l.TryGetRawValue(le, out var rawValue);
+                Assert.True(success);
+                Assert.Equal(1, rawValue);
+            }
+            // empty log message
+            {
+                var le = LogEventInfo.Create(LogLevel.Info, "logger", "message");
+                var success = l.TryGetRawValue(le, out var rawValue);
+                Assert.True(success);
+                Assert.Equal(DBNull.Value, rawValue);
+            }
+
+        }
     }
 }