using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var rnd = new Random();

        for (int i=0; i < 500; i++) {

            Console.WriteLine(RandomString(4, rnd).Mask("##-##"));

        }

    }

    public static string RandomString(int length, Random rnd)

    {

        string text = "abcdefghijklmnoprstuvyzxwq0123456789".ToUpper();

        char[] array = (text).ToCharArray();

        string text2 = string.Empty;

        for (int i = 0; i < length; i++)

        {

            text2 += array[rnd.Next(array.Length)].ToString();

        }

        return text2;

    }

}

​

public static class Extension {

    public static string Mask(this string value, string mask, char substituteChar = '#')

    {

    int valueIndex = 0;

    try

    {

        return new string(mask.Select(maskChar => maskChar == substituteChar ? value[valueIndex++] : maskChar).ToArray());

    }

    catch (IndexOutOfRangeException e)

    {

        throw new Exception("Value too short to substitute all substitute characters in the mask", e);

    }

    }   

}