Question

I’m trying to understand the purpose of TaskCompletionSource and its relation to async/threadless work. I think I have the general idea but I want to make sure my understanding is correct.

I first started looking into the Task Parallel Library (TPL) to figure out if there was a good way to create your own threadless/async work (say you’re trying to improve scalability of your ASP.NET site) plus understanding of the TPL looks like it will be very important in the future (async/await). Which led me to the TaskCompletionSource.

From my understanding it looks like adding TaskCompletionSource to a one of your classes doesn’t really do much in as making your coding async; if you’re still executing sync code then the call to your code will block. I think this is even true of microsoft APIs. For example, say in DownloadStringTaskAsync off of WebClient class, any setup / sync code they are doing initially will block. The code you’re executing has to run on some thread, either the current thread or you will have to spin off a new one.

So you use TaskCompletionSource in your own code when you’re calling other async calls from Microsoft so the client of your classes doesn’t have to create a new thread for your class to not block.

Not sure how Microsoft does their async APIs internally. For example, there is a new async method off of the SqlDataReader for .Net 4.5. I know there is IO Completion Ports. I think it’s a lower level abstraction (C++?) that probably most C# developers won’t use. Not sure if IO completion Ports will work for Database or network calls (HTTP) or if its just used for file IO.

So the question is, am I correct in my understanding correct? Are there certain things I’ve represented incorrectly?

Answer 1

TaskCompletionSource is used to create Task objects that don’t execute code.

They’re used quite a bit by Microsoft’s new async APIs – any time there’s I/O-based asynchronous operations (or other non-CPU-based asynchronous operations, like a timeout). Also, any async Task method you write will use TCS to complete its returned Task.

I have a blog post Creating Tasks that discusses different ways to create Task instances. It’s written from an async/await perspective (not a TPL perspective), but it still applies here.

Also see Stephen Toub’s excellent posts:

• The Nature of TaskCompletionSource
• Mechanisms for Creating Tasks
• await anything; (using TaskCompletionSource to await anything).
• Using Tasks to implement the APM Pattern (creating Begin/End using TaskCompletionSource).