Return Json Exception

The graph facebook API return this error if you dont provide a valid token:

{
"error": {
             "type": "OAuthException",
             "message": "An access token is required to request this resource."
         }
}

In case you make a valid call the json result is returned.

In WCF with .net 4 how can I return the same Json "exception" with this method:

[WebGet(UriTemplate = "{id}")]
public SampleIte开发者_运维技巧m Get(string id)
{
   if (id.Length != 4)
      "how to return here the error since this method return a sample item class"????
   else
      return rep.GetSampleByID(id);
}

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I think I find one way. Returning a stream and managing the returning content type. based on this answer