New custom attribute based on MaybeNullWhen that checks a named out parameter (not just the return value)

[jonopare]

I personally find that the MaybeNullWhenAttribute is very useful with nullable reference types. When a method that returns bool is called, the compiler can check if the return value was true (or false) and then infer whether some nullability constraints have been satisfied (or not).

For example:

static bool TryDoSomething(int number, [MaybeNullWhen(false)] out string message) { ... }

can be used like this without worrying whether message is null:

if (TryDoSomething(5, out var message))
{
    Console.WriteLine(message);
}

However, the void Deconstruct(out bool, out object) pattern doesn't have a boolean return value, and therefore can't make use of the MaybeNullWhenAttribute.

I'd like to propose (if it hasn't already been proposed and I just couldn't find the discussion) a new attribute:

static void Deconstruct(out bool success, [MaybeNullWhenOut(nameof(success))] out object value)

It would enable the following deconstruction to not have to perform null checking when success is true:

// deconstructable is an object whose type has a Deconstruct method with this custom attribute specified
var (success, value) = deconstructable;
if (success) 
{
   Console.WriteLine(value);
}

For me, this would be preferable to changing the signature of Deconstruct to return a out object? value, and subsequently having to perform the null checks myself (or worse, use the ! operator.)

[333fred]

The problem is that the compiler doesn't implement the necessary aliasing analysis that could enable this to work, so you'd have to start by convincing us that is worth the (considerable) effort.

[jonopare]

That makes sense. Thanks for the context — I’ll leave it there. If anyone else thinks it would be worth the considerable effort then I encourage them to champion the idea.

[colejohnson66]

Can a custom analyzer suppress a warning in a spot?

[333fred]

Yes, you can implement a DiagnosticSuppressor.