Deserializing a JSON array of mixed types

问题:

I'm having trouble deserializing a JSON array of mixed types using the DataContractJsonSerializer class. I've spent a bunch of time looking for a solution to no avail, so I thought I'd go ahead and ask here.

Basically, I am getting a JSON string like the one below. I'd like to get the array to deserialize into an List where position 0 has an Int32, position 1 has a String, and position 2 has an instance of my custom class.

[
   2,
   "Mr. Smith",
   {
      "num":169,
      "name":"main street",
      "state":66
   }
]

If I just create a serialize like so:

DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(List<object>))

I actually get an Int32 at position 0 and a String at position 1. However at position 2 I just get a null object.

Does anyone know if what I am trying to do is even possible? I have no control over the structure of the JSON that I'm consuming. I'd like to accomplish this without using third party assemblies if possible.

回答1:

You have to make a class that reproduces the json structure like this:

[DataContract]
public class MyClass {
    [DataMember]
    public int IntMember { get; set; }
    [DataMember]
    public string StringMember { get; set; }
    [DataMember]
    public MyType[] AllTypes { get; set;}
}

[DataContract]
public class MyType {
    [DataMember]
    public int num { get; set; }
    [DataMember]
    public string name { get; set; }
    [DataMember]
    public int state { get; set;}
}

Decorate the class and its properties with the "DataContract" and "DataMember" attributes. Then in your deserializing code use the class you have created as in the following example

var serializer = new DataContractJsonSerializer(typeof(MyClass));
System.IO.StringReader reader = new System.IO.StringReader(jsonData);
System.IO.MemoryStream ms = new System.IO.MemoryStream(Encoding.Default.GetBytes(jsonData));
return serializer.ReadObject(ms) as MyClass;

回答2:

Mike,

The problem is that during deserialization, the JSON deserializer does not know what type to deserialize the element at position 2 to.

You need to provide a "__type" hint. You can get the __type hint for thie particular type by actually serializing it out to JSON in a polymorphic situation, and seeing what type hint gets emitted.

For more info, see http://msdn.microsoft.com/en-us/library/bb412170.aspx. Pay particular attention to the sections "Collections Assigned to Object" and "Preserving Type Information"