using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Foo("{0}, {2}"));  // "random_user_name, random_city"

        Console.WriteLine(Foo("{1}"));       // "random_address"

        Console.WriteLine(Foo("{2}, {1}"));  // "random_city, random_address"

    }

    public static string Foo(string input)

    {

        string user = "random_user_name";

        string address = "random_address";

        string city = "random_city";

​

        return string.Format(input, user, address, city);

    }

}