Make top-level serialization context accessible to custom serializers

BinarySerializer.Test/Custom/CustomTests.cs

@@ -28,11 +28,12 @@ public void TestCustomDateTime()
         public void CustomWithContextTest()
         {
             var expected = new CustomWithContextContainerClass {Value = new CustomWithContextClass()};
+            var context = new CustomWithContextContainerContextClass { Value = "context" };
 
             var serializer = new BinarySerializer();
             var stream = new MemoryStream();
 
-            serializer.Serialize(stream, expected, "context");
+            serializer.Serialize(stream, expected, context);
             stream.Position = 0;
             serializer.Deserialize<CustomWithContextContainerClass>(stream);
         }

BinarySerializer.Test/Custom/CustomWithContextClass.cs

@@ -9,8 +9,10 @@ public void Serialize(Stream stream, BinarySerialization.Endianness endianness,
             BinarySerializationContext serializationContext)
         {
             Assert.AreEqual(typeof(CustomWithContextContainerClass), serializationContext.ParentType);
-            //Assert.AreEqual("context", serializationContext.ParentContext.ParentValue);
-            // TODO check root context
+
+
+            Assert.AreEqual(typeof(CustomWithContextContainerContextClass), serializationContext.ParentContext.ParentContext.ParentType);
+            Assert.AreEqual("context", (serializationContext.ParentContext.ParentContext.ParentValue as CustomWithContextContainerContextClass).Value);
         }
 
         public void Deserialize(Stream stream, BinarySerialization.Endianness endianness,

BinarySerializer.Test/Custom/CustomWithContextContainerContextClass.cs

@@ -0,0 +1,7 @@
+namespace BinarySerialization.Test.Custom
+{
+    public class CustomWithContextContainerContextClass
+    {
+        public string Value { get; set; }
+    }
+}

BinarySerializer/Graph/ValueGraph/RootValueNode.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
@@ -101,6 +102,13 @@ protected override Encoding GetFieldEncoding()
             return EncodingCallback();
         }
 
+        protected override BinarySerializationContext CreateSerializationContext()
+        {
+            var parent = Children.FirstOrDefault()?.Parent;
+            return new BinarySerializationContext(Value, parent?.Value, parent?.TypeNode.Type,
+                null, TypeNode.MemberInfo);
+        }
+
         private static RootTypeNode GetContextGraph(Type valueType)
         {
             lock (ContextCacheLock)

BinarySerializer/Graph/ValueGraph/ValueNode.cs

@@ -960,7 +960,7 @@ private async Task ProcessPaddingAsync(BoundedStream stream, Func<BoundedStream,
             return Convert.ToInt64(binding.ConstValue);
         }
 
-        private BinarySerializationContext CreateSerializationContext()
+        protected virtual BinarySerializationContext CreateSerializationContext()
         {
             var parent = Parent;
             return new BinarySerializationContext(Value, parent?.Value, parent?.TypeNode.Type,