public class A
{
    static void f(string x)
    {
        if (x is null) throw new System.ArgumentNullException(nameof(x));
        if (x !is null) throw new System.ArgumentNullException(nameof(x));
        
        System.Console.WriteLine(x);
    }
    
    static void Main()
    {
        f(null);
    }
}