c# - How does control flow when there is synchronous code in awaited function?

Question

Say in button click event I have:

await someFn()

And in the someFn code, I have got some synchronous code and some asynchronous code.

As per online reading on this topic, they all seem to say that when await is encountered, control immediately goes back the caller (UI thread in this case) while the asynchronous waiting happens.

I haven't come across situation where it is explained what happens when there is some synchronous code just before the async code in the awaited function. Does the synchronous code execute before passing the control to the caller (UI thread)?

I know that- had it been run using await Task.Run(...) instead of await someFn() then it would execute in a separate thread.

标签： c#asynchronousasync-await

Answer 1

It is true that "when await is encountered, control immediately goes back the caller", under the condition that the awaitable will not be completed at the await point. Otherwise (if the awaitable is already completed) the control will stay inside the method, by executing immediately the code that follows the await.

That's all that influences the behavior of the await operator: the result of the IsCompleted property of the awaiter. It doesn't matter if the method that produced the awaitable included synchronous code or not. The await doesn't know, and doesn't care. A synchronous code block inside the someFn will indeed make a difference regarding the responsiveness of your application, but that has nothing to do with the await. To understand why, it may help to rewrite your code in a more verbose but equivalent form. This:

await someFn();

...is equivalent to this:

Task t = someFn();
await t;

The UI of your application will freeze because the someFn() blocks, not because the await t blocks. Unless of course you have created some wicked task-like awaitable type, that blocks when the IsCompleted property of its awaiter is invoked.