How can I use the AsyncCTP with an TFS APM Method (Query.Begin/EndQuery)?

Question

Would like to try using AsyncCTP with TFS. Currently have a long running method that calls RunQuery on a TFS Query instance.

Query exposes the APM methods BeginQuery() and EndQuery(). As I understand it, the recommended approach to wrap these using AsyncCTP is something like: (example from docs)

Task<int>.Factory.FromAsync(stream.BeginRead, stream.EndRead, buffer, offset, count, null);

Further, have wrapped it in an extension method as in the docs so my actual method looks like:

public static Task<WorkItemCollection> RunQueryAsync(this Query query)
{
    if (query== null) 
        throw new ArgumentNullException("Query");

    return Task<WorkItemCollection>.Factory.FromAsync(query.BeginQuery, query.EndQuery, null);
} 

...but this fails to compile. Getting an "invalid argument" intellisense error that, frankly, I can't really understand because the types and format look correct. One possible issue might be that the Query APM methods expect an ICanceleableAsyncResult whereas the Task factory is expecting an IAsyncResult -- but looking at the TFS API, ICanceleableAsyncResult is a specialization of IAsyncResult.

Not sure whether i'm doing it wrong or its just not possible. Would love to be able to do it the AsyncCTP way but may have to go back to the APM pattern -- ugh!

Answer 1

Update: My Nito.AsyncEx library now includes a TeamFoundationClientAsyncFactory type, which can be used instead of rolling your own implementation below.

---

The TFS API is not strictly following the APM pattern because it does not take a state parameter, and this is preventing the built-in TaskFactory.FromAsync from working.

You'll have to write your own FromAsync equivalent, which can be done using TaskCompletionSource:

using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.TeamFoundation.Client;

public static class TfsUtils<TResult>
{
  public static Task<TResult> FromTfsApm(Func<AsyncCallback, ICancelableAsyncResult> beginMethod, Func<ICancelableAsyncResult, TResult> endMethod, CancellationToken token)
  {
    // Represent the asynchronous operation by a manually-controlled task.
    TaskCompletionSource<TResult> tcs = new TaskCompletionSource<TResult>();
    try
    {
      // Begin the TFS asynchronous operation.
      var asyncResult = beginMethod(Callback(endMethod, tcs));

      // If our CancellationToken is signalled, cancel the TFS operation.
      token.Register(asyncResult.Cancel, false);
    }
    catch (Exception ex)
    {
      // If there is any error starting the TFS operation, pass it to the task.
      tcs.TrySetException(ex);
    }

    // Return the manually-controlled task.
    return tcs.Task;
  }

  private static AsyncCallback Callback(Func<ICancelableAsyncResult, TResult> endMethod, TaskCompletionSource<TResult> tcs)
  {
    // This delegate will be invoked when the TFS operation completes.
    return asyncResult =>
    {
      var cancelableAsyncResult = (ICancelableAsyncResult)asyncResult;

      // First check if we were canceled, and cancel our task if we were.
      if (cancelableAsyncResult.IsCanceled)
        tcs.TrySetCanceled();
      else
      {
        try
        {
          // Call the TFS End* method to get the result, and place it in the task.
          tcs.TrySetResult(endMethod(cancelableAsyncResult));
        }
        catch (Exception ex)
        {
          // Place the TFS operation error in the task.
          tcs.TrySetException(ex);
        }
      }
    };
  }
}

You can then use it in extension methods as such:

using System.Threading;
using System.Threading.Tasks;
using Microsoft.TeamFoundation.WorkItemTracking.Client;

public static class TfsExtensions
{
  public static Task<WorkItemCollection> QueryAsync(this Query query, CancellationToken token = new CancellationToken())
  {
    return TfsUtils<WorkItemCollection>.FromTfsApm(query.BeginQuery, query.EndQuery, token);
  }
}