How can I set Headers with Silverlight GET HttpWebRequest?

Question

Making a request to a RESTful service in Silverlight with HttpWebRequest works well so long as I don't add any headers to the request.

As soon as I add a header using code like this

var webReq = (HttpWebRequest)WebRequest.Create(new Uri(_RemoteAddress, "GetProviderMetadata"));
webReq.Method = HttpMethodType.Get;
webReq.Headers["SomeToken"] = "someTokenValue";

I get the exception pasted at the bottom of this question as soon as EndGetResponse() is called. Anyone know why this is? Adding headers to get requests seems to work fine in normal .NET so I'm guessing it's some sort of Silverlight limitation but I can't find any documentation that clarifies.

   {System.NotSupportedException ---> System.NotSupportedException: Specified method is not supported.
  at System.Net.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
  at System.Net.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
  at System.Net.AsyncHelper.<>c__DisplayClass2.<BeginOnUI>b__0(Object sendState)
  --- End of inner exception stack trace ---
  at System.Net.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
  at System.Net.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
  at System.Net.BrowserHttpWebRequest.<>c__DisplayClassd.<InvokeGetResponseCallback>b__b(Object state2)
  at System.Threading._ThreadPoolWaitCallback.WaitCallback_Context(Object state)
  at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack)
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state)}
   [System.NotSupportedException]: {System.NotSupportedException ---> System.NotSupportedException: Specified method is not supported.
  at System.Net.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
  at System.Net.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
  at System.Net.AsyncHelper.<>c__DisplayClass2.<BeginOnUI>b__0(Object sendState)
  --- End of inner exception stack trace ---
  at System.Net.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
  at System.Net.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
  at System.Net.BrowserHttpWebRequest.<>c__DisplayClassd.<InvokeGetResponseCallback>b__b(Object state2)
  at System.Threading._ThreadPoolWaitCallback.WaitCallback_Context(Object state)
  at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack)
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state)}
   _className: "System.NotSupportedException"
   _data: {System.Collections.ListDictionaryInternal}
   _dynamicMethods: null
   _exceptionMethod: null
   _exceptionMethodString: null
   _helpURL: null
   _HResult: -2146233067
   _innerException: {System.NotSupportedException: Specified method is not supported.
  at System.Net.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
  at System.Net.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
  at System.Net.AsyncHelper.<>c__DisplayClass2.<BeginOnUI>b__0(Object sendState)}
   _message: ""
   _remoteStackIndex: 0
   _remoteStackTraceString: null
   _source: "System.Windows"
   _stackTrace: {sbyte[192]}
   _stackTraceString: null
   _xcode: -532462766
   _xptrs: 0
   Data: {System.Collections.ListDictionaryInternal}
   HelpLink: null
   HResult: -2146233067
   InnerException: {System.NotSupportedException: Specified method is not supported.
  at System.Net.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
  at System.Net.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
  at System.Net.AsyncHelper.<>c__DisplayClass2.<BeginOnUI>b__0(Object sendState)}
   Message: ""
   Source: "System.Windows"
   StackTrace: "   at System.Net.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)\r\n   at System.Net.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)\r\n   at Intellidimension.RdfEntity.Service.RemoteEntityServiceProviderClient.getProviderMetadataResponse(IAsyncResult result)\r\n   at System.Net.BrowserHttpWebRequest.<>c__DisplayClassd.<InvokeGetResponseCallback>b__b(Object state2)\r\n   at System.Threading._ThreadPoolWaitCallback.WaitCallback_Context(Object state)\r\n   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)\r\n   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack)\r\n   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state)"
   System.Runtime.InteropServices._Exception.HelpLink: null
   System.Runtime.InteropServices._Exception.Source: "System.Windows"

Answer 1

Silverlight only supports setting headers using the POST method not the GET method. This is due to a limitation in how the TCP/IP stack is implemented in Silverlight. It uses the browser extension APIs instead of going directly against the host OS's APIs.

Answer 2

Specify "Client HTTP" handling before you call you initiate your WebRequest.

Like this:

bool httpResult = WebRequest.RegisterPrefix("http://", WebRequestCreator.ClientHttp);
bool httpsResult = WebRequest.RegisterPrefix("https://", WebRequestCreator.ClientHttp);