using System;

using static System.Console;

public class Program {

    public static void Main() {

        WriteLine("123".Concatenate("456"));

        string.Concat(null, "xxx");

    }

}

​

namespace System {

    public static class StringExt {

        public static string Concatenate(this string str1, string str2) =>  string.Concat(str1, str2);

    }

}