await结合空传播操作符为何触发VS的System.NullReferenceException警告？

Great question—let's unpack exactly what's happening here:

• The null-conditional operator (?.) behavior: When _user is null, _user?.DisposeAsync() doesn't throw right away—it simply returns null. That's the whole point of ?.: it short-circuits property/method calls on null references to avoid immediate exceptions.

• The problem with awaiting null: The await operator needs a valid awaitable object (like Task, ValueTask, or a custom type implementing the awaitable pattern). If you pass null to await, the runtime tries to call GetAwaiter() on that null reference—and that's where the NullReferenceException hits. You can't invoke a method on a null object, even implicitly via await.

• Why removing await fixes the warning: Without await, you're just evaluating the expression _user?.DisposeAsync() which returns null when _user is null—but you're not trying to do anything with that null value. There's no attempt to access members of a null instance, so Visual Studio doesn't see a risk of an exception here.

How to fix this safely #

To avoid the potential exception while keeping the async disposal, rewrite the code to only await when _user isn't null:

if (_user is not null)
{
    await _user.DisposeAsync();
}

Or if you prefer the older null check style:

if (_user != null)
{
    await _user.DisposeAsync();
}

This ensures you only call DisposeAsync() and await it when _user is a valid, non-null instance, eliminating the null await scenario entirely.

内容的提问来源于stack exchange，提问作者Maxim Kitsenko