How does C# 5 async return to main thread?

Question

I was watching a vid about Async CTP and saw that if you call await from e.g. main thread , then the execution will continue from main thread when the work is completed.

e.g

//called from main thread
var result = await SomeAsyncWork();
//this will execute in main thread also
console.writeline(result)

I had the naive impression that there would be a normal call back going on which would be executed on a worker thread.

At some level that must be what is going on since you can wrap normal async methods in a Task of T with Task.FromAsync

but normal async methods will run in worker threads, so how is the callback in the workerthread marshalled back to the main thread?

Answer 1

The callback is attached to the task, using the current task scheduler (that's "current" at the time of awaiting, not current at the time of completion). For a UI thread, the current task scheduler will schedule the callback to be executed within the UI thread... for a thread pool thread, the scheduler will allow it to be executed on any thread pool thread, etc.

Assuming you're awaiting a Task<T>, it's effectively calling Task<T>.ContinueWith(continuationAction, scheduler).

You can await anything with the right methods available, but how BeginAwait schedules the continuation is implementation-specific. I've only mentioned the task-based one because it's probably the most common one. The compiler itself doesn't specify this at all - it assumes that the libraries will do the right thing. All the compiler does is translate "the rest of the method" into a continuation, and pass that into BeginAwait.

Answer 2

The compiler compiles your code differently and splits it so that code after await becomes a callback. There's a detailed explanation to a certain extent here: http://evolpin.wordpress.com/2011/05/02/c-5-await-and-async-in-asp-net/