Question

When I store this class:

class MyClass{
    ...
    public Type SomeType {get;set;} 
    ...
}

SomeType property gets serialized like this:

"SomeType" : {
    "_t" : "RuntimeType"
}

and every subsequent query fails.

I’m using the official C# driver. How do I get it to store the actual Type?
Thanks.

Answer 1

Here’s a sample serializer for System.Type that serializes the name of the Type as a BSON string. This has some limitations in that the Deserialize method fails if the type name is not a system type or in the same assembly, but you could tweak this sample serializer to write the AssemblyQualifiedName instead.

public class TypeSerializer : IBsonSerializer
{
    public object Deserialize(BsonReader reader, Type nominalType, IBsonSerializationOptions options)
    {
        var actualType = nominalType;
        return Deserialize(reader, nominalType, actualType, options);
    }

    public object Deserialize(BsonReader reader, Type nominalType, Type actualType, IBsonSerializationOptions options)
    {
        if (reader.CurrentBsonType == BsonType.Null)
        {
            return null;
        }
        else
        {
            var fullName = reader.ReadString();
            return Type.GetType(fullName);
        }
    }

    public bool GetDocumentId(object document, out object id, out Type idNominalType, out IIdGenerator idGenerator)
    {
        throw new InvalidOperationException();
    }

    public void Serialize(BsonWriter writer, Type nominalType, object value, IBsonSerializationOptions options)
    {
        if (value == null)
        {
            writer.WriteNull();
        }
        else
        {
            writer.WriteString(((Type)value).FullName);
        }
    }

    public void SetDocumentId(object document, object id)
    {
        throw new InvalidOperationException();
    }
}

The trick is to get it registered properly. You need to register it for both System.Type and System.RuntimeType, but System.RuntimeType is not public so you can’t refer to it in your code. But you can get at it using Type.GetType. Here’s the code to register the serializer:

var typeSerializer = new TypeSerializer();
BsonSerializer.RegisterSerializer(typeof(Type), typeSerializer);
BsonSerializer.RegisterSerializer(Type.GetType("System.RuntimeType"), typeSerializer);

I used this test loop to verify that it worked:

var types = new Type[] { typeof(int), typeof(string), typeof(Guid), typeof(C) };
foreach (var type in types)
{
    var json = type.ToJson();
    Console.WriteLine(json);
    var rehydratedType = BsonSerializer.Deserialize<Type>(json);
    Console.WriteLine("{0} -> {1}", type.FullName, rehydratedType.FullName);
}

where C is just an empty class:

public static class C
{
}