Tuesday, May 15, 2012

The Task: Async and Await in a Windows Runtime World

In my last blog post, I covered how to wrap your arms around the Task class and its relationship to the new async and await keywords. I mentioned that the post was focused on the .NET Framework only because the Windows Runtime handles these operations differently. In this post, I’ll cover what those differences are.

Task is a Task is a Task

First, in the Windows Runtime, a Task is a Task … is a Task. You can write your code to return a Task or Task<T> in your Windows 8 Metro applications. If you are going to expose a Windows Runtime (WinRT) component, however, one of the rules is that you must always return a WinRT type. For asynchronous operations, there are four types allowed:

No Result Returns Results
No Progress or Cancellation IAsyncAction IAsyncOperation<TResult>
Supports Progress and/or Cancellation IAsyncActionWithProgress<TProgress> IAsyncOperationWithProgress<TResult, TProgress>

The type you return depends on whether or not you return a result, and whether or not you support checking progress and/or cancellation.

Task is a IAsyncAction or IAsyncOperation<T>

If you don’t support progress or cancellation, returning the necessary type is easy: simply use a Task and return it using one of the extension methods to convert it to the corresponding WinRT type. For example, the following useless piece of code iterates through all of the available integers and returns nothing:

public static IAsyncAction IterateAsync()
   return Task.Run(() =>
      for(int x = Int.MinValue; x < Int.MaxValue; x++) ;

For a more practical example, consider the multiplication method I used in the last blog post. To convert that to a result, I simply do:

public IAsyncOperation<int> Multiply(int a, int b)
   return Task.Run(() => a * b)

That’s fairly simple and straightforward. What about supporting progress and/or cancellation?

Forget Tasks: The 411 on AsyncInfo

The AsyncInfo class is there to assist you with performing asynchronous actions or operations that support cancellation and reporting progress.

public static IAsyncOperationWithProgress<int, double> Multiply(int a, int b)
   return AsyncInfo.Run<IList<long>, double>((token, progress) =>
      Task.Run<int>(() =>
         var result = a*b;
         return result;
      }, token));

Obviously the operation is a bit contrived as the multiplication operation doesn’t take as long, but hopefully this simple example illustrates the point of what is possible. The IAsyncActionWithProgress will work the same way, it simply doesn’t return a result.

There you have it … the scoop on the new async and await keywords and how they behave both with and without the Windows Runtime. Now that you have the basics, head over to Stephen Toub’s blog post and read the far more in depth Diving Deep with WinRT and await.