Is using await on the UI thread the same as a blocking call?

Question

Just reading and thinking aloud: if I used await on a call to a method from within the main UI thread, that would tantamount to being a blocking call, right, since there's no thread that called the UI thread?

For example:

int Main()
{
    // This would be a blocking call, right?
    await PrintTwoMillionTimes("Hello");

    Console.WriteLine("Phew! Done!");
}

Answer 1

No; the whole point of await is that it isn't a blocking call.
The compiler transforms the code after the await into a callback.

As long as the thing you're awaiting is well-behaved (meaning that it won't do its work synchronously), your UI will be fine.

Answer 2

No, absolutely not.

The point of await is that it doesn't block. Assuming the result of PrintTwoMillionTimes behaves sensibly, the async method will return immediately... but it will attach a continuation so that the rest of the method will execute (on the UI thread) when the result completes.

So assuming you've got a working example (currently your method declaration isn't async , and returns int , and you haven't got any return statements...) the flow is:

• The async method starts
• PrintTwoMillionTimes is called, which would start an asynchronous operation, returning something representing the operation (eg a Task )
• The generated code would check whether the operation has already completed:
  • If it's already completed, execution continues
  • Otherwise, a continuation is attached to the task (or whatever is representing the operation) and the async method returns

If you're returning Task or Task<T> from the async method, the return value can be used to indicate when the async method itself has completed (ie after your Console.WriteLine has executed).

This is just a very brief description, of course. You should really read up on async/await in more detail. MSDN is a reasonable starting point.