Make it possible to register a custom FuncLayoutRenderer

src/NLog/LayoutRenderers/FuncLayoutRenderer.cs

@@ -43,14 +43,25 @@ namespace NLog.LayoutRenderers
     /// </summary>
     public class FuncLayoutRenderer : LayoutRenderer, IStringValueRenderer
     {
+        private readonly Func<LogEventInfo, LoggingConfiguration, object> _renderMethod;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="FuncLayoutRenderer"/> class.
+        /// </summary>
+        /// <param name="layoutRendererName">Name without ${}.</param>
+        protected FuncLayoutRenderer(string layoutRendererName)
+        {
+            LayoutRendererName = layoutRendererName;
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="FuncLayoutRenderer"/> class.
         /// </summary>
         /// <param name="layoutRendererName">Name without ${}.</param>
         /// <param name="renderMethod">Method that renders the layout.</param>
         public FuncLayoutRenderer(string layoutRendererName, Func<LogEventInfo, LoggingConfiguration, object> renderMethod)
         {
-            RenderMethod = renderMethod ?? throw new ArgumentNullException(nameof(renderMethod));
+            _renderMethod = renderMethod ?? throw new ArgumentNullException(nameof(renderMethod));
             LayoutRendererName = layoutRendererName;
         }
 
@@ -60,9 +71,11 @@ public FuncLayoutRenderer(string layoutRendererName, Func<LogEventInfo, LoggingC
         public string LayoutRendererName { get; set; }
 
         /// <summary>
-        /// Method that renders the layout. 
+        /// Method that renders the layout.
+        ///
+        /// This public property will be removed in NLog 5.
         /// </summary>
-        public Func<LogEventInfo, LoggingConfiguration, object> RenderMethod { get; }
+        public Func<LogEventInfo, LoggingConfiguration, object> RenderMethod => _renderMethod;
 
         /// <summary>
         /// Format string for conversion from object to string.
@@ -73,7 +86,7 @@ public FuncLayoutRenderer(string layoutRendererName, Func<LogEventInfo, LoggingC
         /// <inheritdoc />
         protected override void Append(StringBuilder builder, LogEventInfo logEvent)
         {
-            var value = GetValue(logEvent);
+            var value = RenderValue(logEvent);
             var formatProvider = GetFormatProvider(logEvent, null);
             builder.AppendFormattedValue(value, Format, formatProvider);
         }
@@ -85,16 +98,21 @@ private string GetStringValue(LogEventInfo logEvent)
         {
             if (Format != MessageTemplates.ValueFormatter.FormatAsJson)
             {
-                object value = GetValue(logEvent);
+                object value = RenderValue(logEvent);
                 string stringValue = FormatHelper.TryFormatToString(value, Format, GetFormatProvider(logEvent, null));
                 return stringValue;
             }
             return null;
         }
 
-        private object GetValue(LogEventInfo logEvent)
+        /// <summary>
+        /// Render the value for this log event
+        /// </summary>
+        /// <param name="logEvent">The event info.</param>
+        /// <returns>The value.</returns>
+        protected virtual object RenderValue(LogEventInfo logEvent)
         {
-            return RenderMethod.Invoke(logEvent, LoggingConfiguration);
+            return _renderMethod?.Invoke(logEvent, LoggingConfiguration);
         }
     }
 }

src/NLog/LayoutRenderers/LayoutRenderer.cs

@@ -244,7 +244,7 @@ protected CultureInfo GetCulture(LogEventInfo logEvent, CultureInfo layoutCultur
         /// <typeparam name="T"> Type of the layout renderer.</typeparam>
         /// <param name="name"> Name of the layout renderer - without ${}.</param>
         public static void Register<T>(string name)
-            where T: LayoutRenderer
+            where T : LayoutRenderer
         {
             var layoutRendererType = typeof(T);
             Register(name, layoutRendererType);
@@ -280,8 +280,17 @@ public static void Register(string name, Func<LogEventInfo, object> func)
         public static void Register(string name, Func<LogEventInfo, LoggingConfiguration, object> func)
         {
             var layoutRenderer = new FuncLayoutRenderer(name, func);
-
-            ConfigurationItemFactory.Default.GetLayoutRenderers().RegisterFuncLayout(name, layoutRenderer);
+
+            Register(layoutRenderer);
+        }
+
+        /// <summary>
+        /// Register a custom layout renderer with a callback function <paramref name="layoutRenderer"/>. The callback receives the logEvent and the current configuration.
+        /// </summary>
+        /// <param name="layoutRenderer">Renderer with callback func</param>
+        public static void Register(FuncLayoutRenderer layoutRenderer)
+        {
+            ConfigurationItemFactory.Default.GetLayoutRenderers().RegisterFuncLayout(layoutRenderer.LayoutRendererName, layoutRenderer);
         }
     }
 }

tests/NLog.UnitTests/LayoutRenderers/FuncLayoutRendererTests.cs

@@ -0,0 +1,85 @@
+// 
+// Copyright (c) 2004-2020 Jaroslaw Kowalski <[email protected]>, Kim Christensen, Julian Verdurmen
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without 
+// modification, are permitted provided that the following conditions 
+// are met:
+// 
+// * Redistributions of source code must retain the above copyright notice, 
+//   this list of conditions and the following disclaimer. 
+// 
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution. 
+// 
+// * Neither the name of Jaroslaw Kowalski nor the names of its 
+//   contributors may be used to endorse or promote products derived from this
+//   software without specific prior written permission. 
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE 
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF 
+// THE POSSIBILITY OF SUCH DAMAGE.
+// 
+
+using NLog.LayoutRenderers;
+using System;
+using NLog.Config;
+using Xunit;
+
+namespace NLog.UnitTests.LayoutRenderers
+{
+    public class FuncLayoutRendererTests : NLogTestBase
+    {
+        [Fact]
+        public void RegisterCustomFuncLayoutRendererTest()
+        {
+            // Arrange
+            var funcLayoutRenderer = new MyFuncLayoutRenderer("the-answer-new");
+
+            // Act
+            LayoutRenderer.Register(funcLayoutRenderer);
+
+            LogManager.Configuration = XmlLoggingConfiguration.CreateFromXmlString(@"<nlog throwExceptions='true'>
+                <targets>
+                    <target name='debug' type='Debug' layout= 'TheAnswer=${the-answer-new:Format=D3}' /></targets>
+                <rules>
+                    <logger name='*' minlevel='Debug' writeTo='debug' />
+                </rules>
+            </nlog>");
+
+            var logger = LogManager.GetCurrentClassLogger();
+            logger.Debug("test1");
+
+            // Assert
+            AssertDebugLastMessage("debug", "TheAnswer=042");
+        }
+
+        private class MyFuncLayoutRenderer : FuncLayoutRenderer
+        {
+            /// <inheritdoc />
+            public MyFuncLayoutRenderer(string layoutRendererName) : base(layoutRendererName)
+            {
+            }
+
+            #region Overrides of FuncLayoutRenderer
+
+            /// <inheritdoc />
+            protected override object RenderValue(LogEventInfo logEvent)
+            {
+                return 42;
+            }
+
+            #endregion
+        }
+    }
+}