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);
	}
}

//https://pt.stackoverflow.com/q/189606/101