await with null propagation System.NullReferenceException

Question

I have the following code:

await _user?.DisposeAsync();

Visual Studio highlights this code, saying 'Possible NullReferenceException'

by the way, without await Visual Studio doesn't show this warning Why NullReferenceException is possible here?

Answer 1

await null will throw a NullReferenceException. Therefore if _user is null, then _user?.DisposeAsync() will return null, and the await will throw.

You can do:

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

(you might need a local copy of _user if it might change between reads)

or:

await (_user?.DisposeAsync() ?? ValueTask.CompletedTask);

(Prior to .NET 5, you will need:)

await (_user?.DisposeAsync().AsTask() ?? Task.CompletedTask);

Answer 2

Async methods return a Task that can be awaited. If _user is null then you would not be returning a Task but null instead

Answer 3

await is expecting a result. If _user is null then the result will be null, hence the NullReferenceException.