Consuming WCF Data Services Service Operator (WebGet) async from Silverlight

Question

Having a lot of problems trying to consume a simple service operator in a WCF Data Service from Silverlight. I've verified the following service operator is working by testing it in the browser:

[WebGet]
public IQueryable<SecurityRole> GetSecurityRolesForUser(string userName) {
  string currentUsername = HttpContext.Current.User.Identity.Name;

  // if username passed in, verify current user is admin and is getting someone else's permissions
  if (!string.IsNullOrEmpty(userName)) {
    if (!SecurityHelper.IsUserAdministrator(currentUsername))
      throw new DataServiceException(401, Properties.Resources.RequiestDeniedInsufficientPermissions);
  } else // else nothing passed in, so get the current user's permissions
    userName = currentUsername;

  return SecurityHelper.GetUserRoles(userName).AsQueryable<SecurityRole>();
}

However no matter how I try using different methods I've found in various online resources, I've been unable to consume the data. I've tried using the BeginExecute() method on boht the DataServiceContext and DataServiceQuery, but I keep getting errors or no data returned in the EndExecute method. I've got to be doing something simple wrong... here's my SL code:

private void InitUserSecurityRoles() {
  MyEntities context = new MyEntities(new Uri("http://localhost:9999/MyService.svc"));
  context.BeginExecute<SecurityRole>(new Uri("http://localhost:9999/MyService.svc/GetSecurityRolesForUser"), OnComplete, context);

  DataServiceQuery<SecurityRole> query = context.CreateQuery<SecurityRole>("GetSecurityRolesForUser");
  query.BeginExecute(OnComplete, query);
}

private void OnComplete(IAsyncResult result) {
  OnDemandEntities context = result.AsyncState as OnDemandEntities;
  var x = context.EndExecute<SecurityRole>(result);
}

Any tips? I'm at a loss right now on how to properly consume a custom service operator from Silverlight (or even sync using my unit test project) from a OData service. I've also verified via Fiddler that I'm passing along the correct authentication stuff as well, even going to far as explicitly set the credentials. Just to be safe, I even removed the logic from the service operator that does the security trimming.

Answer 1

Got it working thanks to @kaevans (http://blogs.msdn.com/b/kaevans):

private void InitUserSecurityRoles() {
  DataServiceContext context = new DataServiceContext(new Uri("http://localhost:9999/MyService.svc"));
  context.BeginExecute<SecurityRole>(new Uri("/GetSecurityRolesForUser", UriKind.Relative),
    (result) => {
      SmartDispatcher.BeginInvoke(
        () => {
          var roles = context.EndExecute<SecurityRole>(result);

          UserSecurityRoles = new List<SecurityRole>();
          foreach (var item in roles) {
            UserSecurityRoles.Add(item);
          }
        });
    }, null);
}

I had to create the SmartDispatcher because this is happening in a ViewModel. Otherwise I could have just used the static Dispatcher.BeginInvoke(). Couldn't get the roles variable to insert into my UserSecurityRoles (type List) directly for sone reason using various techniques, so I just dropped down to adding it manually (code isn't called often nor is it a collection exceeding more than 10 items max... most are <5).