#nullable enable

using System;

​

public interface IBase {

}

​

public class Z : IBase {

}

​

public interface IX {

}

​

public class X : IX, IBase {

}

​

public class Program

{

    public static void Main()

    {

        TestIsNot(null);

        TestIsNot(new Z());

        TestIsNot(new X());

    }

    public static void TestIsNot(object? obj){

        if (obj is not IX notX){

            System.Console.Out.WriteLine($"Not x but not null? {notX is not null}");

        }

    }

}