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
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