using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string test = Sprintf("Testing %s and %s", new string[]{"This", "That"});

        Console.WriteLine(test);

    }

    public static string Sprintf(string format, string[] args)

    {

        Regex needle = new Regex("(%s)");

        int i = 0;

        while (needle.IsMatch(format))

        {

            if (i >= args.Length)

            {

                break;

            }

​

            format = needle.Replace(format, args[i], 1);

            i++;

        }

        return format;

    }   

}