Hello,

This one is real simple, run this Silverlight4 example with the ContentType property commented out and you'll get back a response from from my service in xml. Now uncomment the property and run it and you'll get an ProtocolViolationException, what should happen is the service returns JSON formatted data.

HttpWebRequest wreq = 
(HttpWebRequest)WebRequestCreator.ClientHttp.Create(new Uri("http://stephenpattenconsulting.com/Services/GetFoodDescriptionsLookup(2100)"));

//wreq.ContentType = "application/json";

wreq.BeginGetResponse((cb) =>
{
    HttpWebRequest rq = cb.AsyncState as HttpWebRequest;
    HttpWebResponse resp = rq.EndGetResponse(cb) as HttpWebResponse; // Exception
    StreamReader rdr = new StreamReader(resp.GetResponseStream());
    string result =  rdr.ReadToEnd(); 
    rdr.Close();
}, wreq);

EDIT: While perusing SO, I noticed similar posts, like this one Why am I getting ProtocolViolationException when trying to use HttpWebRequest? and this one How do I use HttpWebRequest with GET method

EDIT: The WCF 4 end point is 'adapted' from this link http://geekswithblogs.net/michelotti/archive/2010/08/21/restful-wcf-services-with-no-svc-file-and-no-config.aspx.

I can use Fiddler's request builder to to construct the proper requests and changing the content type does yield the correct results.