NLogBeginScopeParser - Reduce allocation for BeginScope with message template (#451)

Author: snakefoot

src/NLog.Extensions.Logging/Logging/NLogBeginScopeParser.cs

@@ -95,6 +95,28 @@ public static IDisposable CaptureScopeProperties(IReadOnlyList<KeyValuePair<stri
                 object scopeObject = scopePropertyList;
 
                 if (scopePropertyList.Count > 0 && NLogLogger.OriginalFormatPropertyName.Equals(scopePropertyList[scopePropertyList.Count - 1].Key))
+                {
+                    scopePropertyList = ExcludeOriginalFormatProperty(scopePropertyList);
+                }
+
+                return CreateScopeProperties(scopeObject, scopePropertyList);
+            }
+
+            private static IReadOnlyList<KeyValuePair<string, object>> ExcludeOriginalFormatProperty(IReadOnlyList<KeyValuePair<string, object>> scopePropertyList)
+            {
+                if (scopePropertyList.Count == 2 && !NLogLogger.OriginalFormatPropertyName.Equals(scopePropertyList[0].Key))
+                {
+                    scopePropertyList = new[] { scopePropertyList[0] };
+                }
+                else if (scopePropertyList.Count <= 2)
+                {
+#if NET451
+                    scopePropertyList = new KeyValuePair<string, object>[0];
+#else
+                    scopePropertyList = Array.Empty<KeyValuePair<string, object>>();
+#endif
+                }
+                else
                 {
                     var propertyList = new List<KeyValuePair<string, object>>(scopePropertyList.Count - 1);
                     for (var i = 0; i < scopePropertyList.Count; ++i)
@@ -110,7 +132,7 @@ public static IDisposable CaptureScopeProperties(IReadOnlyList<KeyValuePair<stri
                     scopePropertyList = propertyList;
                 }
 
-                return CreateScopeProperties(scopeObject, scopePropertyList);
+                return scopePropertyList;
             }
 
             public static IDisposable CaptureScopeProperties(IEnumerable scopePropertyCollection, ExtractorDictionary stateExtractor)

test/NLog.Extensions.Logging.Tests/CustomBeginScopeTest.cs

@@ -43,6 +43,17 @@ public void TestNonSerializableSayHi()
             // Assert.Equal("Earth People", scopeString); <-- Bug https://github.com/aspnet/Logging/issues/893
         }
 
+        [Fact]
+        public void TestNonSerializableSayHiToEarth()
+        {
+            var runner = GetRunner<CustomBeginScopeTestRunner>();
+            var target = new Targets.MemoryTarget { Layout = "${message} ${mdlc:Planet}. Welcome to the ${mdlc:Galaxy}" };
+            ConfigureNLog(target);
+            var scopeString = runner.SayHiToEarth().Result;
+            Assert.Single(target.Logs);
+            Assert.Equal("Hi Earth. Welcome to the Milky Way", target.Logs[0]);
+        }
+
         [Fact]
         public void TestNonSerializableSayNothing()
         {
@@ -82,6 +93,16 @@ public async Task<string> SayHi()
                 }
             }
 
+            public async Task<string> SayHiToEarth()
+            {
+                using (var scopeState = _logger.BeginScope("{Planet} in {Galaxy}", "Earth", "Milky Way"))
+                {
+                    await Task.Yield();
+                    _logger.LogInformation("Hi");
+                    return scopeState.ToString();
+                }
+            }
+
             public async Task SayNothing()
             {
                 using (var scopeState = _logger.BeginScope(new Dictionary<string,string>()))