using System;

public class Program

{

    public static void Main()

    {

        string a = null;

        Console.WriteLine(a.DoSomething());

    }

}

public static class StringExtension{

    private static readonly object Lock = new object();

        public static string DoSomething(this string test)

        {

            lock(Lock){

                DoSomethingMore(test);

            }

            return "Ala ma kota";

        }

        public static string DoSomethingMore(this string test)

        {

            lock(Lock){

                DoSomethingMore(test);

            }

            return "Ala ma kota";

        }

}