Task chaining without TaskCompletionSource?

Question

I'm converting some async/await code to chained tasks, so I can use it in the released framework. The await code looks like this

public async Task<TraumMessage> Get() {
  var message = await Invoke("GET");
  var memorized = await message.Memorize();
  return memorized;
}

where

Task<TraumMessage> Invoke(string verb) {}
Task<TraumMessage> Memorize() {}

I was hoping to chain Invoke and Memorize to return the task produced by Memorize, but that results in a Task<Task<TraumMessage>. The solution i've ended up is a TaskCompletionSource<TraumMessage> as my signal:

public Task<TraumMessage> Get() {
  var completion = new TaskCompletionSource<TraumMessage>();
  Invoke("GET").ContinueWith( t1 => {
     if(t1.IsFaulted) {
       completion.SetException(t1.Exception);
       return;
     }
     t1.Result.Memorize().ContinueWith( t2 => {
       if(t2.IsFaulted) {
         completion.SetException(t2.Exception);
         return;
       }
       completion.SetResult(t2.Result);
     });
  });
  return completion.Task;
}

Is there a way to accomplish this without the TaskCompletionSource?

Answer 1

Yes, the framework comes with a handy Unwrap() extension method for exactly what you want.

Invoke("GET").ContinueWith( t => t.Result.Memorize() ).Unwrap();

If you're doing cancellation then you'll need to pass cancel tokens into the appropriate places, obviously.

Answer 2

I think that's pretty much the only way to accomplish what you want. Chaining disparate Tasks together isn't supported by the continuation APIs, so you have to resort to using a TaskCompletionSource like you have to coordinate the work.

I don't have the Async CTP installed on this machine, but why don't you take a look at the code with a decompiler (or ILDASM if you know how to read IL) to see what it's doing. I bet it does something very similar to your TCS code under the covers.