How to mitigate "Access to modified closure" in cases where the delegate is called directly

Question

My understanding is that the "Access to modified closure" warning is there to warn me about accessing local variables from a delegate when the delegate might be stored and called later or called on a different thread so that the local variable isn't actually available at the time of actual code execution. This is sensible of course.

But what if I am creating a delegate that I know is going to be called immediately in the same thread? The warning then is not needed. For example the warning is generated in the code:

delegate void Consume();

private void ConsumeConsume(Consume c)
{
    c();
}

public int Hello()
{
    int a = 0;

    ConsumeConsume(() => { a += 9; });

    a = 1;

    return a;
}

There can be no problem here since ConsumeConsume always calls the function immediately. Is there any way around this? Is there some way to annotate the function ConsumeConsume to indicate the ReSharper that the delegate will be called immediately?

Interestingly, when I replace the ConsumeConsume(() => { a += 9; }); line with:

new List<int>(new[] {1}).ForEach(i => { a += 9; });

which does the same thing, no warning is generated. Is this just an in-built exception for ReSharper or is there something I can do similarly to indicate that the delegate is called immediately?

I am aware that I can disable these warnings but that is not a desired outcome.

Answer 1

Install the JetBrains.Annotations package with NuGet: https://www.nuget.org/packages/JetBrains.Annotations

Mark the passed in delegate with the InstantHandle attribute.

private void ConsumeConsume([InstantHandle] Consume c)
{
    c();
}

From InstantHandle's description:

Tells code analysis engine if the parameter is completely handled when the invoked method is on stack. If the parameter is a delegate, indicates that delegate is executed while the method is executed. If the parameter is an enumerable, indicates that it is enumerated while the method is executed.

Source: https://www.jetbrains.com/help/resharper/Reference__Code_Annotation_Attributes.html

If you don't want to add the whole package to your project, it's enough to just add the attribute yourself, although it's hacky in my opinion.

namespace JetBrains.Annotations
{
    [AttributeUsage(AttributeTargets.Parameter)]
    public class InstantHandleAttribute : Attribute { }
}