How are async void event handlers called in C#?

问题

If I declare my event handlers as async void, will they be called synchronously or asynchronously by the .NET framework?

I.e., given the following code:

async void myButton_click(object sender, EventArgs e) {
   do stuff
   await longRunning()
   do more stuff
}

Can I be sure that the "do stuff" line will be executed on the GUI thread?

回答1:

Event handlers will be called synchronously and doesn't wait(await) till the event handler completes, but waits till the event handler returns.

If previous sentence was confusing enough, I'll try to explain it clear. Asynchronous methods completes when all the await points are executed and the end of the method body has reached or any return statement is executed(Ignoring the exception). But asynchronous method returns as soon as you hit the first await statement for the Task which is not yet completed. In other words asynchronous method can return several times but can complete only once.

So now we know when does a asynchronous method completes and returns. Event handler will assume your method has completed as soon as it returns not when it actually completes.

As soon as your event handler reaches first await statement, it will return, if there are more methods attached to same event handler, it will continue executing them without waiting for the asynchronous method to complete.

Yes, do stuff will be executed in UI thread if the UI thread fires the event and yes do more stuff will also be executed in UI thread as long as longRunning().ConfigureAwait(false) isn't called.

回答2:

They will be invoked just as any other non-async-await method is invoked:

Click(this, EventArgs.Empty);

Because this specific event handler is an async method the call would run synchronously until an await is reached and the rest would be a continuation. That means that do stuff is executed synchronously on the GUI thread. The caller then moves on without the knowledge that the async operation hasn't completed yet.

do more stuff would also be executed on the GUI thread, but for a different reason. The SynchronizationContext in a GUI environment makes sure the continuations would be posted to the single GUI thread, unless you explicitly tell it not to with await longRunning().ConfigureAwait(false)

来源：https://stackoverflow.com/questions/27228476/how-are-async-void-event-handlers-called-in-c

标签

.net

event-handling

async-await