Alpha Numeric Counter

Hi,

Here is a little Alpha numeric counter I have written that will either continue counting or will return the next value.


public string AlphaNum(string val)
        {
            StringBuilder sb = new StringBuilder(val);
            bool incNext = false;
            for (int n = sb.Length; n > 0; n--)
            {
                if (n == sb.Length || incNext)
                {
                    char nextChar;
                    char letter = Convert.ToChar(sb.ToString(n - 1, 1));
                    if (letter == '9')
                    {
                        incNext = true;
                        nextChar = 'A';
                    }
                    else if (letter == 'Z')
                    {
                        if (n - 2 > -1)
                        {
                            if (sb.ToString(n - 2, 1) != "Z")
                            {
                                incNext = true;
                                nextChar = '0';
                            }
                            else
                            {
                                incNext = true;
                                nextChar = 'Z';
                            }
                        }
                        else
                        {
                            incNext = true;
                            nextChar = 'Z';
                        }
                    }
                    else
                    {
                        nextChar = (char)(((int)letter) + 1);
                        incNext = false;
                    }
                    sb.Remove(n - 1, 1).Insert(n - 1, nextChar.ToString());
                }
            }
            return sb.ToString();
        }