Can ReSharper [CanBeNull] and [NotNull] attributes be applied to an Action or Func argument?

Question

ReSharper has a suite of code annotations which are useful for explicitly expressing code intent that can be used by the IDE. Two of the most useful annotations are the [CanBeNull] and [NotNull] attributes, which can be used on Constructors, Properties, and Methods, like this:

[CanBeNull]
private Foo DoSomething([NotNull] string text)
{
    // ...
}

This is a long shot, but is there any way these attributes can be assigned to an Action or Func parameter?

I understand that the following code is illegal (because type arguments are not a valid target for attributes), but is there an alternative way of expressing this?

private void DoSomething(Action<[NotNull]string> processText)
{
    ///...
}

Answer 1

You can do this if you're willing to create a custom delegate type:

    delegate void TextProcessor([NotNull] string text);

    delegate void NullableTextProcessor([CanBeNull] string text);

    private void DoSomething([NotNull] TextProcessor processText)
    {
        // ...
    }

    private void DoSomethingNull([NotNull] NullableTextProcessor processText)
    {
        // ...
    }

Unfortunately, CanBeNull doesn't seem to give warnings in lambda syntax:

But you might just want to wait for C# 8's nullable/non-nullable reference types.