code for keygenerator

class KeyGenerator
{
    public KeyGenerator()
    {
        chars = new List<char>();
        generated = new List<string>();
        for (int i = 0; i < 10; i++)
            chars.Add((char)('0' + i));
        for (char c = 'A'; c <= 'Z'; c++)
            chars.Add(c);
        random = new Random();
    }

    private List<char> chars;
    private List<string> generated;
    Random random;
    private List<string> keys;
    public string GetNext(int len)
    {
        StringBuilder result = new StringBuilder();
        bool duplicate = true;
        while(duplicate)
        {
            result.Clear();
            for (int i = 0; i < len; i++)
            {
                int index = random.Next(chars.Count);
                result.Append(chars[index]);
            }
            if (!generated.Contains(result.ToString()))
                duplicate = false;
        }
        return result.ToString();
    }
}

KeyGenerator keyGenerator = new KeyGenerator();
for (int i = 0; i < 5; i++)
   Console.WriteLine(keyGenerator.GetNext(16));