Increments a string like Excel [A ... AA ... AAA ... AAAA]

StringIncrementable.cs

class StringIncrementable
{
    public static string Increment(string strFrom)
    {
        char[] letters = new char[strFrom.Length + 1];
        int i;
        
        for (i = 0; i < strFrom.Length; i++)
            letters[i] = strFrom[i];

        for (i = letters.Length - 2; i >= 0; i--)
        {
            if (letters[i] == 'Z')
            {
                letters[i] = 'A';
                if (i > 0 && letters[i - 1] != 'Z')
                {
                    letters[i - 1]++;
                    break;
                }   
                else if (i == 0)
                {
                    for (int j = 0; j < letters.Length; j++)
                        letters[j] = 'A';
                    break;
                }

            }
            else
            {
                letters[i]++;
                break;
            }
        }

        return new string(letters).Trim('\0');
    }
}