Waiting on an IAsyncResult method that waits on another IAsyncResult (Chaining)

Question

(can only use .NET 3.5 stock, so no Tasks, no Reactive Extensions)

I have, what I thought to be a simple case, but I'm baffled at it.

The short of it is that, I'm returning BeginGetRequestStream's IAsyncResult to the caller of BeginMyOperation(), and I want to really send back the IAsyncResult of BeginGetResponse, which is called when the EndGetRequestStream is called.

So I'm wondering, how do I

      public IAsyncResult BeginMyOperation(...)
      {
            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(requestUri);
            webRequest.Method = "POST";

            // This is the part, that puzzles me. I don't want to send this IAsyncResult back.
            return webRequest.BeginGetRequestStream(this.UploadingStreamCallback, state);
       }

      // Only want this to be called when the EndGetResponse is ready.
      public void EndMyOperation(IAsyncResult ar)
      {

      }

      private IAsyncResult UploadingStreamCallback(IAsyncResult asyncResult)
      {
            using (var s = state.WebRequest.EndGetRequestStream(asyncResult))
            {
                using (var r = new BinaryReader(state.Request.RequestData))
                {
                    byte[] uploadBuffer = new byte[UploadBufferSize];
                    int bytesRead;
                    do
                    {
                        bytesRead = r.Read(uploadBuffer, 0, UploadBufferSize);

                        if (bytesRead > 0)
                        {
                            s.Write(uploadBuffer, 0, bytesRead);
                        }
                    }
                    while (bytesRead > 0);
                }
            }

            // I really want to return this IAsyncResult to the caller of BeginMyOperation
            return state.WebRequest.BeginGetResponse(new AsyncCallback(state.Callback), state);
        }

Answer 1

I think the easiest way to solve this is to use Task wrappers. In particular, you can finish a TaskCompletionSource when BeginGetResponse completes. Then just return the Task for that TaskCompletionSource. Note that Task implements IAsyncResult, so your client code won't have to change.

Personally, I would go a step further:

1. Wrap BeginGetRequestStream in a Task (using FromAsync).
2. Create a continuation for that Task that processes the request and wraps BeginGetResponse in a Task (again, using FromAsync).
3. Create a continuation for that second Task that completes the TaskCompletionSource.

IMHO, exceptions and result values are more naturally handled by Tasks than IAsyncResult.

Answer 2

The thing you're trying to do is doable, but you need to create a new implementation of IAsyncResult (something like "CompositeResult" that watches the first IAsyncResult, then kicks off the 2nd call).

However, this task is actually far easier using the Reactive Extensions - in that case you'd use Observable.FromAsyncPattern to convert your Begin/End methods into a Func that returns IObservable (which also represents an async result), then chain them using SelectMany:

IObservable<Stream> GetRequestStream(string Url);
IObservable<bool> MyOperation(Stream stream);

GetRequestStream().SelectMany(x => MyOperation(x)).Subscribe(x => {
    // When everything is finished, this code will run
});