WCF 3.5, AsyncBridge. Wrap in async-await

WCF 3.5, AsyncBridge. Wrap in async-await

At work I'm currently stuck in 3.5, but we are using the asyncbridge for
async-await. We're using alot of old WCF async calls, and I want to wrap
this into the new async-await pattern.
I'm wrapping this as follows:
// async is wrong
public /*async*/ Task<ScannedDocumentResult>
GetScannedDocumentsTask(String assignmentId)
{
TaskCompletionSource<ScannedDocumentResult> tcs = new
TaskCompletionSource<ScannedDocumentResult>();
EventHandler<GetScannedDocumentsCompletedEventArgs> handler = null;
handler = (o, e) =>
{
if (e.UserState != tcs)
return;
if (e.Error != null)
tcs.SetException(e.Error);
else if (e.Cancelled)
tcs.SetCanceled();
else
tcs.SetResult(e.Result);
GetScannedDocumentsCompleted -= handler;
};
GetScannedDocumentsCompleted += handler;
GetScannedDocumentsAsync(assignmentId, tcs);
return tcs.Task;
}
The following are genereted in the 3.5 WCF proxy:
GetScannedDocumentsAsync GetScannedDocumentsCompleted
GetScannedDocumentsEventArgs
Something tells me that this can be done alot cleaner, have I missed
something cruical?
Also, will this method execute async at all? Compiling with the async
operator just generates an error.
Cheers, Stian