Array to new string

Question

I'm making a webshop for school and had a quick question. I'm trying to write a code that generates a random coupon code and it actually works (did some extreme programming in Console Application), but it's simply not efficient.

static void Main(string[] args)
    {
        Random r = new Random();
        string ALphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        int size = 4;

        char[] code1 = new char[size]
        char[] code2 = new char[size]
        char[] code3 = new char[size]
        for (int i = 0; i < size; i++)
        {
            code1[i] = Alphabet[r.Next(Alphabet.Length)];
            code2[i] = Alphabet[r.Next(Alphabet.Length)];
            code3[i] = Alphabet[r.Next(Alphabet.Length)];
        }

        string code4 = new string(code1);
        string code5 = new string(code2);
        string code6 = new string(code3);

        Console.WriteLine(code4 + " - " + code5 + " - " + code6);
        Console.ReadLine();
    }

This works.. somehow. But I would like to see it more efficient, because when I want to generate 100 coupons... this isn't really the way to do that.

I did see something on joining strings, use string.Insert to get that " - " in between and loop it multiple times, but I couldn't get a clear tutorial on how to do that with... well this kind of code.

Anyone got a efficient and (preferable) easy solution?

=======

UPDATE!

this does end up in a database eventually

Answer 1

Code is not quite good, but for school app will play I guess )

    static string GenerateCoupon(Random r)
    {
        string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        int size = 4;

        char[] code1 = new char[size];
        char[] code2 = new char[size];
        char[] code3 = new char[size];
        for (int i = 0; i < size; i++)
        {
            code1[i] = Alphabet[r.Next(Alphabet.Length)];
            code2[i] = Alphabet[r.Next(Alphabet.Length)];
            code3[i] = Alphabet[r.Next(Alphabet.Length)];
        }

        string code4 = new string(code1);
        string code5 = new string(code2);
        string code6 = new string(code3);

        return string.Format("{0}-{1}-{2}", code4, code5, code6);
    }

    static void Main(string[] args)
    {
        Random rnd = new Random();
        for (int i = 0; i < 100;i++ )
            Console.WriteLine(GenerateCoupon(rnd));
         Console.ReadLine();

    }

Answer 2

You could be using a StringBuilder for this:

    StringBuilder sb = new StringBuilder();

    Random r = new Random();
    string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    int size = 16;

    for (var i = 0; i < size; i++)
    {
        sb.Append(Alphabet[r.Next(Alphabet.Length)]);
    }                 

    Console.WriteLine(sb.ToString());

If you want less code you can make use of a GUID and format it.

Guid.NewGuid().ToString("N").Substring(0, 16);

Update, just saw you needed some formatting between each part of the coupon, so I changed it a litle bit:

    StringBuilder sb = new StringBuilder();

    Random r = new Random();
    string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    int pieces = 3, pieceSize = 4;

    for (var i = 0; i < pieces; i++)
    {
        if(i != 0)
            sb.Append(" - ");

        for (var j = 0; j < pieceSize; j++)
        {
            sb.Append(Alphabet[r.Next(Alphabet.Length)]);
        }
    }

    Console.WriteLine(sb.ToString());