await keyword blocks main thread

Question

So I have the following code

private async void button1_Click(object sender, EventArgs e) {     await DoSomethingAsync();      MessageBox.Show("Test"); }  private async Task DoSomethingAsync() {     for (int i = 0; i < 1000000000; i++)     {         int a = 5;     }; // simulate job      MessageBox.Show("DoSomethingAsync is done");      await DoSomething2Async(); }  private async Task DoSomething2Async() {     for (int i = 0; i < 1000000000; i++)     {         int a = 5;     } // simulate job      MessageBox.Show("DoSomething2Async is done"); } 

Until both MessageBoxes are shown the main thread is block (I mean the application itself is frozen). There is obviously something wrong with my code and I can't figure out what. I've never used async/await before. this is my first attempt.

EDIT:

Actually what i want to do is to start Execution of DoSomethingAsync asynchronously so that when button is clicked the MessageBox.Show("Test"); would execute even though the DoSomethingAsync is incomplete.

Answer 1

I think you misunderstand what async means. It doesn't mean that the method runs in another thread!!

An async method runs synchronously until the first await, then returns a Task to the caller (unless it's async void, then it returns nothing). When the task that is being awaited completes, execution resumes after the await, usually on the same thread (if it has a SynchronizationContext).

In your case, the Thread.Sleep is before the first await, so it's executed synchronously, before control is returned to the caller. But even if it was after the await, it would still block the UI thread, unless you specifically configured the the awaiter not to capture the synchronization context (using ConfigureAwait(false)).

Thread.Sleep is a blocking method. If you want an async equivalent, use await Task.Delay(3000), as suggested in Sriram Sakthivel's answer. It will return immediately, and resume after 3 seconds, without blocking the UI thread.

It's a common misconception that async is related to multithreading. It can be, but in many cases it's not. A new thread is not implicitly spawned just because the method is async; for a new thread to spawn, it has to be done explicitly at some point. If you specifically want the method to run on a different thread, use Task.Run.