Why Elvis (?.) operator doesn't work with async-await?

Question

Let's have a code like this (fragment of App.xaml.xs):

public class MethodClass
{
    public async Task Job()
    {
        Debug.WriteLine("Doing some sob");
        await Task.Delay(1);
    }
}

public MethodClass MyClass = null;

protected async override void OnLaunched(LaunchActivatedEventArgs e)
{
    await MyClass?.Job(); // here goes NullreferenceException
    MyClass?.Job(); // works fine - does nothing

Why Elvis operator doesn't work with async-await? Am I missing something?

Answer 1

The way await is translated is that first, GetAwaiter() is called on the awaited object (in your case, a Task). It then does some other complicated things, but those are not relevant here:

await MyClass?.Job();

Compiles to:

var awaiter = MyClass?.Job().GetAwaiter();
// more code

Since Task.GetAwaiter() is an instance method and you're calling it with a null Task, you get a NullReferenceException.

---

As a curiosity, it is possible to await a null awaitable, as long as its GetAwaiter() is an extension method that accepts null:

public class NullAwaitable { }

public static class Extensions
{
    public static TaskAwaiter GetAwaiter(this NullAwaitable _)
        => Task.CompletedTask.GetAwaiter();
}

public class MethodClass
{
    public NullAwaitable Job() => new NullAwaitable();
}

MethodClass MyClass = null;

await MyClass?.Job(); // works fine