How do I use HttpWebRequest GET method w/ ContentType="application/json"

Question

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.

#if DEBUG
                Address = "http://localhost.:55437/Services/GetFoodDescriptionsLookup(2100)";
#else
                Address = "http://stephenpattenconsulting.com/Services/GetFoodDescriptionsLookup(2100)";
#endif

Uri httpSite = new Uri(Address);

HttpWebRequest wreq = 
(HttpWebRequest)WebRequestCreator.ClientHttp.Create(httpSite);

//wreq.ContentType = "application/json"; // Wrong
wreq.Accept = "application/json"; // Right

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);

New exception

System.NotSupportedException was unhandled by user code Message="" StackTrace: at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state) at System.Net.Browser.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at com.patten.silverlight.ViewModels.WebRequestLiteViewModel.b_0(IAsyncResult cb) at System.Net.Browser.BrowserHttpWebRequest.<>c_DisplayClassd.b__b(Object state2) at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx) at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() at System.Threading.ThreadPoolWorkQueue.Dispatch() at System.Threading.ThreadPoolWaitCallback.PerformWaitCallback() InnerException: System.NotSupportedException Message=Specified method is not supported. StackTrace: at System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult) at System.Net.Browser.BrowserHttpWebRequest.<>c_DisplayClass5.b_4(Object sendState) at System.Net.Browser.AsyncHelper.<>c_DisplayClass2.b__0(Object sendState) InnerException:

WORKING NOW

The exception that I was getting was due to a hack I use to get Fiddler to show the loop-back adapter i.e. http://localhost.:55437/Services/GetFoodDescriptionsLookup(2100), notice the extra DOT after the word localhost.

That was it!

I know what everyone is thinking, this could have been solved it I would have included some of these details.. like the debug flag that changes the URI, I removed most of the code to make it easier to read on SO, thinking that the problem was in the networking stack. Hard lesson(s) learned.

Thanks to everyone who took the time to get involved with this, both on stackoverflow and offline.

For completeness I have added the jQuery function that had me thinking this whole time I needed to be setting "content-type" instead of "accept" when in the SL4 application. (READ THE DOCS!)

function CallService(serviceUrl, data, callback) {
    $.ajax({
        url: serviceUrl,
        data: data,
        dataType: 'json',
        contextType: "application/json", 
        cache: false,
        success: callback,
        error: function (msg) {
            alert("Request Failed!");
        }
    });
}

Please post the request & response headers from Fiddler; also, what exactly is the exception? I mean, what is the message and inner exception. — Sander
Does the new exception in your edit come with the exact same code you posted above? Because I tried it and it worked without any problems, so I am surprised. Can you post a Fiddler log of your Silverlight application's call when this happens? (i.e. not a manual request created in Fiddler). An exception would arise at that location if you were using this web service cross-domain (because there is no policy file present) but not the exception you posted, so I assume that is not the issue; just to be sure - is your client application also on the same domain? (stephenpattenconsulting.com) — Sander
Yes, the exception is thrown when creating the HttpWebResponse. — Stephen Patten

Glenn Block Glenn Block · Accepted Answer · 2011-01-12T06:12:18

Stephen, setting the content type is to allow SENDING JSON, not receiving it. There are several ways to do this. You can support both JSON and XML or only JSON. If all you want to do is support JSON then you can do this by setting the ResponseFormat property on the WebGet attribute for the service operation.

If you want to support both JSON and XML then you need to either enable automatic format selection on the service and send an accept header of "application/json", otherwise you can use a format string parameter in the query string and have the service use the WebOperationContext.OutgoingResponse.Format property.

Here's a post that covers various ways to do this from the server side. If you go the route of supporting automatic format selection then you need to set the accept header of the HttpWebRequest to "application/json"

How do I use HttpWebRequest GET method w/ ContentType="application/json"

4 Answers