public static void Main()
Console.WriteLine("Hello World");
Console.WriteLine("+791233584478".AddPrefix("W: "));
Console.WriteLine("+791233584478".AddPrefix(""));
Console.WriteLine("".AddPrefix("W: "));
public static class StringExtensions
public static string AddPrefix(this string str, string prefix)
return string.IsNullOrWhiteSpace(str) || string.IsNullOrEmpty(prefix) ? str : prefix + str;