Using TPL with existing async APIs

Question

I want to use TPL with a existing API, RestSharp to be specific, so I can use continuations.

But this means I have to wrap a API that doesn't take the classical .NET approach to async, but instead implements callbacks. Take some code like this:

var client = new RestClient("service-url");
var request = new RestRequest();

client.ExecuteAsync<List<LiveTileWeatherResponse>>(request, 
    (response) =>
    {
        ...
    });

So here I want to wrap the ExecuteAsync in TPL, if it's possible. But I can't for the life of me, figure out how to do it.

Any ideas?

Answer 1

TPL provides TaskCompletionSource class which allows you to expose pretty much anything as a Task. By calling SetResult or SetException , you can cause the task to succeed or fail. In your example, you could probably do something like:

static Task<T> ExecuteTask<T>(this RestClient client, RestRequest request)
{
    var tcs = new TaskCompletionSource<T>();
    client.ExecuteAsync<T>(request, response => tcs.SetResult(response));
    return tcs.Task;
}

You can then use it:

var task = client.ExecuteTask<List<LiveTileWeatherResponse>>(request);
foreach (var tile in task.Result)
{}

Or, if you want to chain tasks:

var task = client.ExecuteTask<List<LiveTileWeatherResponse>>(request);
task.ContinueWith(
    t => 
    {
        foreach (var tile in t.Result)
        {}
    }
);

You can read more about TaskCompletionSource at http://blogs.msdn.com/b/pfxteam/archive/2009/06/02/9685804.aspx

Answer 2

This was a major pain point for me when learning the TPL as well.

What you're looking for is TaskCompletionSource . When you create a TaskCompletionSource , it creates a special Task object (accessible by the TaskCompletionSource.Task property) which only completes when you call the SetResult or SetException methods on the associated TaskCompletionSource .

This post explains how to wrap APM operations with the TPL (and also Rx). See also this gist demonstrating an APM operation wrapped in the TPL.