具有继承功能的反序列化Newtonsoft JSON.NET无法正常工作

Question

(Using Newtonsoft JSON.NET)

Hi I've got a problem when deserializing a list of events where Event is the base type and eg ResourceEvent is a subclass. If I just do a serialize and then deserialize everything works ok and the list contains the ResourceEvents - however I store those using EF7 into a jsonb column in postgres.

When I get the JSON payload from postgres it has reordered (but JSON legal) the properties. When I try to deserialize using Newtonsoft JSON.net it only give me a list with base types Event and not the subclass ResourceEvent.

I've added the two highly stripped examples where the differences is where the "Type" property is on different places. The ordering of other properties has also been scrambeled in the classes. I've got no annotations on the classes at all.

This payload successfully deseralizes:

json
{
    "$type": "System.Collections.Generic.List`1[[Event, Tests]], mscorlib",
    "$values": [
        {
            "$type": "ResourceConfigurationEvent, Tests",
            /* Properties */
            "Resources": {
                /* Properties */                
            },
            "Type": 1            
        }
    ]
}

This payload unsuccessfully deserializes:

json
{
    "$type": "System.Collections.Generic.List`1[[Event, Tests]], mscorlib",
    "$values": [
        {
            "Type": 1,            
            "$type": "ResourceConfigurationEvent, Tests",
            /* Properties */
            "Resources": {
                /* Properties */                
            },
        }
    ]
}

Answer 1

Json.Net normally expects the $type metadata to be the first property of each object for best efficiency in deserialization. If the $type does not appear first, then Json.Net assumes it isn't there. That is why you are getting different results when the properties are reordered.

Fortunately, Json.Net provides a MetadataPropertyHandling setting to allow it to cope with this situation. If you set MetadataPropertyHandling to ReadAhead it should solve your problem. Note that this will have an impact on performance.

Here is a code sample from the documentation.

string json = @"{
  'Name': 'James',
  'Password': 'Password1',
  '$type': 'MyNamespace.User, MyAssembly'
}";

object o = JsonConvert.DeserializeObject(json, new JsonSerializerSettings
{
    TypeNameHandling = TypeNameHandling.All,
    // $type no longer needs to be first
    MetadataPropertyHandling = MetadataPropertyHandling.ReadAhead
});

User u = (User)o;

Console.WriteLine(u.Name);
// James