Is there a pattern for wrapping existing BeginXXX/EndXXX async methods into async tasks?

Question

Is there a pattern for wrapping existing BeginXXX/EndXXX async methods into async tasks?

I'm thinking primarily of Async Socket programming.

 

Answer 1

Yes.  In .NET 4, the Task Parallel Library includes a built in wrapper for the APM pattern (Begin/End): Task.Factory.FromAsync.  For example, if you wanted to create a task for a call to Stream's BeginRead/EndRead method, you could do:

Stream s = ...;
byte [] buffer = ...;
Task<int> numBytesRead = Task<int>.Factory.FromAsync(s.BeginRead, s.EndRead, buffer, 0, buffer.Length, null);
// or with await
int numBytesRead = await Task<int>.Factory.FromAsync(s.BeginRead, s.EndRead, buffer, 0, buffer.Length, null);

Under the covers, FromAsync is just built on top of TaskCompletionSource<TResult>.  A simple version of FromAsync for this read example would look something like:

var tcs = new TaskCompletionSource<TResult>();
s.BeginRead(buffer, 0, buffer.Length, iar =>
{
    try { tcs.SetResult(s.EndRead(iar)); }
    catch(Exception exc) { tcs.SetException(exc); }
}, null);
Task<int> numBytesRead = tcs.Task;

I hope that helps.