Why is AsyncContext needed when using async/await

问题:

I'm calling an async method within my console application. I don't want the app to quit shortly after it starts, i.e. before the awaitable tasks complete. It seems like I can do this:

internal static void Main(string[] args)
{
    try
    {
        Task.WaitAll(DoThisAsync());
    }
    catch (Exception ex)
    {
        Console.Error.WriteLine(ex);
        throw;
    }
}

internal static async Task DoThisAsync()
{
    //...
}

But according to Stephen Cleary's article it seems like I can't do that and should instead create some kind of context for the async to return to when it's done (e.g. AsyncContext).

The code above works though, and it returns on the main thread after Task.WaitAll(DoThisAsync());, so why do I need to use a custom context?

回答1:

It's not required; it's just my preference.

You can synchronously block on a task within Main (using Wait/Result/WaitAll). The semantics are slightly different; in particular, if the async code fails, then Wait/Result/WaitAll will wrap the exception in an AggregateException, while AsyncContext does not.

Also, AsyncContext treats the main thread specially; instead of sending continuations to the thread pool, it will send them back to that main thread (by default; you can always use ConfigureAwait(false) to avoid this). I find this useful if I'm writing a "proof of concept" console app, because AsyncContext behaves very similarly to the UI contexts.

But at the end of the day, it's just a matter of preference.