The .NET DefaultValue attribute is now respected

- Properties with the .NET component model DefaultValue and matching
  values will no longer be serialized unless EmitDefaults is on
  (previously only properties with null or zero were ignored).
- Moved the default exclusion mechanism from
  DefaultExclusiveObjectGraphVisitor to the traversal strategy, since
  visitors have no access to property attributes (it seems more correct
  as well).

YamlDotNet.RepresentationModel/Serialization/FullObjectGraphTraversalStrategy.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Globalization;
 using System.Linq;
 using System.Reflection;
@@ -15,6 +16,7 @@ public class FullObjectGraphTraversalStrategy : IObjectGraphTraversalStrategy
 	{
 		protected readonly Serializer serializer;
 		private readonly int maxRecursion;
+        public bool EmitDefaults { get; set; }
 
 		public FullObjectGraphTraversalStrategy(Serializer serializer, int maxRecursion)
 		{
@@ -87,7 +89,7 @@ protected virtual void Traverse(object value, Type type, IObjectGraphVisitor vis
 					}
 
 					// This is a nullable type, recursively handle it with its underlying type.
-					// Not that if it contains null, the condition above already took care of it
+					// Note that if it contains null, the condition above already took care of it
 					Traverse(value, underlyingType, visitor, currentDepth);
 					break;
 			}
@@ -186,12 +188,26 @@ protected virtual void SerializeProperties(object value, Type type, IObjectGraph
 		{
 			visitor.VisitMappingStart(value, type, typeof(string), typeof(object));
 
+			var props = TypeDescriptor.GetProperties(type);
+
 			foreach (var property in GetTraversableProperties(type))
 			{
 				var propertyValue = property.GetValue(value, null);
 				var propertyType = property.PropertyType;
 				var propertyName = GetPropertyName(type, property);
 
+				if (!EmitDefaults)
+				{
+					if ((propertyType.IsValueType && propertyValue == Activator.CreateInstance(propertyType)) || propertyValue == null)
+						continue;
+
+					var defaultAttr = (DefaultValueAttribute)props[property.Name].Attributes[typeof (DefaultValueAttribute)];
+
+					if (defaultAttr != null && propertyValue.Equals(defaultAttr.Value))
+						continue;
+				}
+
+
 				if(visitor.EnterMapping(propertyName, typeof(string), propertyValue, propertyType))
 				{
 					Traverse(propertyName, typeof(string), visitor, currentDepth);

YamlDotNet.RepresentationModel/Serialization/IObjectGraphTraversalStrategy.cs

@@ -14,5 +14,10 @@ public interface IObjectGraphTraversalStrategy
 		/// <param name="type">The static type of the graph.</param>
 		/// <param name="visitor">An <see cref="IObjectGraphVisitor"/> that is to be notified during the traversal.</param>
 		void Traverse(object graph, Type type, IObjectGraphVisitor visitor);
+
+	    /// <summary>
+	    /// Defines whether the traversal should include elements which contain the default value (e.g. null)
+	    /// </summary>
+	    bool EmitDefaults { get; set; }
 	}
 }

YamlDotNet.RepresentationModel/Serialization/Serializer.cs

@@ -170,11 +170,6 @@ private IObjectGraphVisitor CreateEmittingVisitor(Emitter emitter, Serialization
 				emittingVisitor = new AnchorAssigningObjectGraphVisitor(emittingVisitor, eventEmitter, anchorAssigner);
 			}
 
-			if ((options & SerializationOptions.EmitDefaults) == 0)
-			{
-				emittingVisitor = new DefaultExclusiveObjectGraphVisitor(emittingVisitor);
-			}
-
 			return emittingVisitor;
 		}
 
@@ -194,14 +189,20 @@ private IEventEmitter CreateEventEmitter(Emitter emitter, SerializationOptions o
 
 		private IObjectGraphTraversalStrategy CreateTraversalStrategy(SerializationOptions options)
 		{
+		    IObjectGraphTraversalStrategy strategy;
+
 			if ((options & SerializationOptions.Roundtrip) != 0)
 			{
-				return new RoundtripObjectGraphTraversalStrategy(this, 50);
+				strategy = new RoundtripObjectGraphTraversalStrategy(this, 50);
 			}
 			else
 			{
-				return new FullObjectGraphTraversalStrategy(this, 50);
+				strategy = new FullObjectGraphTraversalStrategy(this, 50);
 			}
+
+		    strategy.EmitDefaults = (options & SerializationOptions.EmitDefaults) != 0;
+
+		    return strategy;
 		}
 	}
 }

YamlDotNet.RepresentationModel/YamlDotNet.RepresentationModel.csproj

@@ -80,7 +80,6 @@
     <Compile Include="Serialization\ChainedEventEmitter.cs" />
     <Compile Include="Serialization\ChainedObjectGraphVisitor.cs" />
     <Compile Include="Serialization\CustomSerializationObjectGraphVisitor.cs" />
-    <Compile Include="Serialization\DefaultExclusiveObjectGraphVisitor.cs" />
     <Compile Include="Serialization\DefaultObjectFactory.cs" />
     <Compile Include="Serialization\DeserializationContext.cs" />
     <Compile Include="Serialization\DeserializationOptions.cs" />

YamlDotNet.UnitTests/RepresentationModel/SerializationTests.cs

@@ -70,6 +70,7 @@ public string Nothing
 
 			private int myInt = 1234;
 
+			[DefaultValue(666)]
 			public int MyInt
 			{
 				get
@@ -769,6 +770,22 @@ public void DeserializationOfNullWorksInJson()
 			}
 		}
 
+		[Fact]
+		public void DefaultValueAttributeIsRespected()
+		{
+			var serializer = new Serializer();
+
+			using (StringWriter buffer = new StringWriter())
+			{
+				X original = new X { MyInt = 666 };
+				serializer.Serialize(buffer, original, typeof(X));
+
+				Console.WriteLine(buffer.ToString());
+
+				Assert.False(buffer.ToString().Contains("MyInt"));
+			}
+		}
+
 		//[Fact]
 		//public void DeserializationIgnoresUnknownProperties()
 		//{