Serialization Fail

interface MyInterface
{
    int Property1 { get; set; }

    int Property2 { get; set; }
}

[Serializable]
[DataContract]
public MyObject1 : MyInterface
{
    [DataMember(Name = "p1")
    public int Property1 { get; set; }

    [DataMember(Name = "p2")
    public int Property2 { get; set; }

    public MyObject1() {}

    public MyObject1(int a, int b) { this.Property1 = a; this.Property2 = b; }
}

public MyObject2, ISerializable
{
   public string P1 { get; set; }

   public MyInterface M1 { get; set; }

   public MyObject2() {}

   public MyObject2(SerializationInfo info, StreamingContext context) {}

   public void GetObjectData(SerializationInfo info, StreamingContext context)
   {
       info.AddValue("p1", this.P1);
       info.AddValue("m1", this.M1);
   }
}


//--------
// Code to Serialize looks like this:

MyObject2 obj = new MyObject2();
obj1.p1 = 1;
obj1.m1 = new MyObject1(1, 2);

DataContractJsonSerializer serializer = new DataContractJsonSerializer(obj.GetType());
MemoryStream ms = new MemoryStream();
serializer.WriteObject(ms, obj);
return Encoding.Default.GetString(ms.ToArray());