NullTarget - Avoid string-allocation upfront when FormatMessage = true

snakefoot · snakefoot · commit d4c7f7c36b8d · 2025-09-28T19:52:22.000+02:00
diff --git a/src/NLog/Targets/NullTarget.cs b/src/NLog/Targets/NullTarget.cs
@@ -33,6 +33,8 @@
 
 namespace NLog.Targets
 {
+    using System.Text;
+
     /// <summary>
     /// Discards log messages. Used mainly for debugging and benchmarking.
     /// </summary>
@@ -54,13 +56,36 @@ namespace NLog.Targets
     [Target("Null")]
     public sealed class NullTarget : TargetWithLayout
     {
+        /// <summary>
+        /// Gets the number of times this target has been called.
+        /// </summary>
+        /// <docgen category='Layout Options' order='10' />
+        public int LogEventCounter => _logEventCounter;
+        private int _logEventCounter;
+
         /// <summary>
         /// Gets or sets a value indicating whether to perform layout calculation.
         /// </summary>
         /// <remarks>Default: <see langword="false"/></remarks>
         /// <docgen category='Layout Options' order='10' />
         public bool FormatMessage { get; set; }
 
+        /// <summary>
+        /// Gets the last message rendered by this target.
+        /// </summary>
+        /// <remarks>Requires <see cref="FormatMessage"/> = <see langword="true"/></remarks>
+        /// <docgen category='Layout Options' order='10' />
+        public string LastMessage => _lastMesageBuilder?.ToString() ?? string.Empty;
+        private StringBuilder? _lastMesageBuilder;
+
+        /// <summary>
+        /// Gets the last LogEvent rendered by this target.
+        /// </summary>
+        /// <remarks>Requires <see cref="FormatMessage"/> = <see langword="true"/></remarks>
+        /// <docgen category='Layout Options' order='10' />
+        public LogEventInfo? LastLogEvent => _lastLogEvent;
+        private LogEventInfo? _lastLogEvent;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="NullTarget" /> class.
         /// </summary>
@@ -84,9 +109,14 @@ public NullTarget(string name) : this()
         /// <param name="logEvent">The logging event.</param>
         protected override void Write(LogEventInfo logEvent)
         {
+            _logEventCounter++;
+
             if (FormatMessage)
             {
-                RenderLogEvent(Layout, logEvent);
+                _lastLogEvent = logEvent;
+                var stringBuilder = _lastMesageBuilder ?? (_lastMesageBuilder = new StringBuilder(128));
+                stringBuilder.Length = 0;
+                Layout?.Render(logEvent, stringBuilder);
             }
         }
     }
diff --git a/tests/NLog.UnitTests/LayoutRenderers/AllEventPropertiesTests.cs b/tests/NLog.UnitTests/LayoutRenderers/AllEventPropertiesTests.cs
@@ -120,7 +120,7 @@ public void IncludeScopeProperties()
 
             var logFactory = new LogFactory().Setup().LoadConfiguration(builder =>
             {
-                builder.ForLogger().WriteTo(new DebugTarget("debug") { Layout = testLayout }).WithAsync();
+                builder.ForLogger().WriteTo(new NullTarget("debug") { Layout = testLayout, FormatMessage = true }).WithAsync();
             }).LogFactory;
 
             var ev = new LogEventInfo(LogLevel.Info, null, null, "{pi:0}", new object[] { 3.14159265359 });
@@ -130,7 +130,7 @@ public void IncludeScopeProperties()
                 logger.Debug(ev);
             }
 
-            var target = logFactory.Configuration.AllTargets.OfType<DebugTarget>().First();
+            var target = logFactory.Configuration.AllTargets.OfType<NullTarget>().First();
             logFactory.Shutdown();  // Flush
 
             var result = target.LastMessage;

Original file line number	Diff line number	Diff line change
`@@ -120,7 +120,7 @@ public void IncludeScopeProperties()`
`120`	`120`
`121`	`121`	`var logFactory = new LogFactory().Setup().LoadConfiguration(builder =>`
`122`	`122`	`{`
`123`		`- builder.ForLogger().WriteTo(new DebugTarget("debug") { Layout = testLayout }).WithAsync();`
	`123`	`+ builder.ForLogger().WriteTo(new NullTarget("debug") { Layout = testLayout, FormatMessage = true }).WithAsync();`
`124`	`124`	`}).LogFactory;`
`125`	`125`
`126`	`126`	`var ev = new LogEventInfo(LogLevel.Info, null, null, "{pi:0}", new object[] { 3.14159265359 });`
`@@ -130,7 +130,7 @@ public void IncludeScopeProperties()`
`130`	`130`	`logger.Debug(ev);`
`131`	`131`	`}`
`132`	`132`
`133`		`- var target = logFactory.Configuration.AllTargets.OfType<DebugTarget>().First();`
	`133`	`+ var target = logFactory.Configuration.AllTargets.OfType<NullTarget>().First();`
`134`	`134`	`logFactory.Shutdown(); // Flush`
`135`	`135`
`136`	`136`	`var result = target.LastMessage;`