NotNullWhenAttribute for Tuples?

[Pentadome]

I have a method called
TryGetCustomAttribute<TAttribute>(MemberInfo member)
That returns the Tuple: (bool Success, TAttribute? Attribute)
How do i indicate that Attribute is not null when Success is true?

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Document Details

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• ID: 17290c4d-73f0-5ae1-e8ba-c039fe9fe9ca
• Version Independent ID: 59e0e3bb-b2e5-3ced-72d9-42e641b3897f
• Content: Upgrade APIs for nullable reference types with attributes that define expectations for null values
• Content Source: docs/csharp/nullable-attributes.md
• Product: dotnet-csharp
• Technology: csharp-null-safety
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

/cc @cartermp Do you know the best way to annotate a method like this?

[cartermp]

I don't think it's possible to apply this attribute to elements of a tuple type. Might be good to file this as a suggestion under dotnet/csharplang.

[BillWagner]

Thanks @cartermp I've transferred the issue to the csharplang repo.

[canton7]

I'm not arguing for or against this request, but I will just point out what's currently available.

Firstly, note that this already exists. This means you can call member.GetCustomAttribute<SomeAttribute>(), and it'll give you back the SomeAttribute if found, or null if it wasn't.

Secondly, the normal pattern for TryXXX methods is:

public static bool TryGetCustomAttribute<T>(MemberInfo member, [MaybeNullWhen(false)] out T attribute);

From personal experience, I've found this is easier to use than returning a (bool success, T? value): for starters, you can use it in a single-line if:

if (TryGetCustomAttribute<SomeAttribute>(member, out var attribute))
{
   // use attribute
}

[chucker]

Secondly, the normal pattern for TryXXX methods is:
[..]
From personal experience, I've found this is easier to use than returning a (bool success, T? value):

That's definitely 1) the correct pattern where possible, and 2) easier to use, but it doesn't work for async.

The closest approximation for async I can think of is indeed a tuple, and for that, we could indeed use a "not null when tuple field A is true".

[DavidArno]

@canton7

From personal experience, I've found this is easier to use than returning a (bool success, T? value): for starters, you can use it in a single-line if...

You can with tuples too:

if (TryGetCustomAttribute<SomeAttribute>(member) is (true, {} attribute))
{
   // use attribute
}

Some people like out parameters. For those that don't, there really is no need to use them any more as tuples and pattern matching serve the same purpose.

And because that {} attribute tests that the value isn't null and assigns it to a non-nullable variable, there's arguably no real need a NotNullWhenAttribute attribute.