asp.net web api - json serialization of interface member resulting in mystifying error

Question

I'm using ASP.NET Web API and I'm trying to return an instance of a class that looks like this:

public class Whatever
{
  public int MyInt {get; set;}
  public IFoo MyFoo {get; set;}
}

When this instance gets serialized to JSON, the IFoo member (which could be an instance of any of a number of classes implementing IFoo) gets serialized oddly. The property name is written, but its value is a huge chunk of HTML containing an error message like this:

You must write an attribute 'type'='object' after writing the attribute with local name '__type'.

Is this the framework telling me that it doesn't know how to serialize an instance cast as an interface? Or something else?

My experiments suggest that the problem is that the member is expressed as an interface. When expressed as concrete type, the error disappears. But there are scenarios where I need to be able to assign multiple types to the same member of the class. What do I do then? — David
– David, Commented May 2, 2012 at 15:39

David · Accepted Answer · 2012-05-02 16:05:13Z

3

It appears that this error occurs whenever the framework tries to serialize a member whose type is an interface. Apart from changing the member so that its type is concrete, I can find no other way around this problem.

answered May 2, 2012 at 16:05

David

16.2k23 gold badges96 silver badges156 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Aliostad Over a year ago

Yes, you cannot serialize an interface. You have to provide KnownTypes and all that. avoid exposing interfaces in your Web API.

David Over a year ago

Thanks for your response. I've never come across the KnownTypes thing before. As you say, it's probably not great design to have an interface member in a view model class anyway.

Collectives™ on Stack Overflow

asp.net web api - json serialization of interface member resulting in mystifying error

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related