using System;

using System.Diagnostics.CodeAnalysis;

​

public enum E {

    a, b

}

​

public static class EExtensions {

    public static void f(this E? e) {

        Console.WriteLine("test");

    }

}

​

public class Program

{   

    public static void Main()

    {

        E? e = null;

        e.f();

    }

}