using System;

using System.Text.RegularExpressions;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var x = "x";

        Console.WriteLine(strings.Format("Hello World {1} {2} {3} {0}", x, "a", "b", "c"));

    }

}

​

public static class strings

{

    public static string Format(this string value, params object[] args)

    {

        // foreach (var arg in args) Console.WriteLine(arg);

        // if (!args.Any()) return value;

        // return Regex.Matches(value, @"\{\d+\}").Count == args.Length ? string.Format(value, args) : value;

        return Regex.Replace(value, @"\{\d+\}", match =>

        {

            var digits = match.Value.Substring(1, match.Value.Length - 2);

            if (int.TryParse(digits, out var index) && index < args.Length)

            {

                Console.WriteLine(digits + " = " + args[index]);

                return args[index]?.ToString() ?? string.Empty;

            }

​

            Console.WriteLine("a" + match.Value);

            return match.Value;

        });

    }

}