Here's a small recursive code for increasing a string. It's for increasing through Excel columns, so it also increases strings like "AD" or "ZZ". Use the method IncString:

private string IncString(string s)
{
if (String.IsNullOrEmpty(s)) return "A";
if (s[s.Length - 1] == 'Z')
{
if (s.Length == 1) return "AA";
else return IncString(s.Substring(0, s.Length - 1)) + "A";
}
else
{
if (s.Length == 1) return IncChar(s[0]).ToString();
else return s.Substring(0, s.Length - 1) + IncChar(s[s.Length - 1]).ToString();
}
}

    private char IncChar(char c)
    {
        return (char)((int)c + 1);
    }