Custom serializer of enumerable type

[darkl]

Hi, Thanks for the great work on msgpack-cli!

I'm having an issue in the following scenario: I want to implement a custom serializer for a type that I don't own. (it belongs to a dll that I can't modify)

The type implements the IEnumerable interface:

public class Person : IEnumerable<Person>
{
    public string Name { get; set; }

    private IEnumerable<Person> Children { get; set; }

    public IEnumerator<Person> GetEnumerator()
    {
        return Children.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

I've implemented a custom serializer and registered it:

SerializationContext serializationContext = 
    new SerializationContext();

serializationContext.Serializers.Register(new PersonSerializer());

When I try serialize an instance of the Person class, it works.

But if I try to serialize an array of objects which one of them is an instance of the Person class, I get a NullReferenceException:

object[] array = new object[] {new Person {Name = "Joe"}, 3};

MessagePackSerializer<object[]> context =
    serializationContext.GetSerializer<object[]>();

byte[] packed = context.PackSingleObject(array); // throws NullReferenceException

Thanks for your help,
Elad

[yfakariya]

Hi, Elad.
Thank you for reporting. It is bug related to collection type property which does not have Add method. I'll fix it.

[yfakariya]

@darkl

I have a qustion. Will you deserialize object array from binary? It is hard to implement because serialized msgpack binary does not contain enough information to restore the types, so the serializer currently returns MessagePackObject array.
I'm fixing NRE problem locally, but I worry that you might face this problem after all.

[darkl]

Hi @yfakariya
Thanks for your reply.
I am not interested in deserializing an object array (I am interested in deserializing an array of a custom type, such as "Person").

Note that it is possible to restore the types by adding additional data to the serialized data, such as the assembly qualified name of the types. (You can also add an optimization and check if the static type of the serialized instance equals to the runtime type of the instance)
This makes the output heavier, so you can make it optional. This is the approach Newtonsoft.Json took:
https://github.com/JamesNK/Newtonsoft.Json/blob/bcd6982419c6165ed2c606eb9994c1aa6bce3735/Src/Newtonsoft.Json/TypeNameHandling.cs
https://github.com/JamesNK/Newtonsoft.Json/blob/f6c74bdf3c75240f943d46249ca86e9319e4e2c8/Src/Newtonsoft.Json.Tests/Documentation/Samples/Serializer/SerializeTypeNameHandling.cs

Anyway, I don't need this, but it may be useful for others.

[yfakariya]

@darkl

Thank you for suggestion and useful information. Adding type information is great idea, but it hurts interoperability for other implementation. I think the most advantage of msgpack is interoperability, not speed. I guess we have to construct a large eco system with non .NET worlds.
So, embedding type identifier is great, but I think its priority is not very high.

[yfakariya]

@darkl

I have just fix this issue. Could you test the package? If the issue is solved, I will publish it.

https://drive.google.com/file/d/0B4q-BkPBs6Mjc3RfLXgzYWxicVE/edit?usp=sharing

[darkl]

Hi,
Thanks for the quick fix!
I tested the package. It still throws NullReferenceException.

[darkl]

Ok, I checked it a bit:
If I create my own SerializationContext, it throws NullReferenceException. If I use SerializationContext.Default it works fine.

I rather work with my own SerializationContext than with the static one.

[yfakariya]

@darkl

Sorry, I had to notice you about issue #26 which I had found when I had been fixing this issue.
The NullReferenceException in fixed package was caused because of issue #26. If you use object, msgpack-cli uses internal object serializer to serialize objects dynamically, and it uses PackerUnpackerExtensions.Pack<T>, and the method used brand-new SerializationContext incorrectly. The method now uses SerializationContext.Default, so you could avoid NRE with SerializationContext.Default. But my recommended aproach to avoid this bizzare behavior is just avoiding object as following.

SerializationContext serializationContext = new SerializationContext();
serializationContext.Serializers.Register( new PersonSerializer() );
// Avoid object to avoid bizarre behavior related to issue #26.
MessagePackSerializer<Person> context = serializationContext.GetSerializer<Person>();
var array = new Person { Name = "Joe" }; 
byte[] packed = context.PackSingleObject( array );
var unpacked = context.UnpackSingleObject( packed );

Assert.That( unpacked.Name, Is.EqualTo( "Joe" ) );
Assert.That( unpacked.Children, Is.Null );

[darkl]

Thanks, I'll work around the issue (in one of the ways you mentioned) until 0.5 is released.

[darkl]

Can you publish the current version to NuGet gallery?

[yfakariya]

I've just publish it.