C#中 Excel列字母與數字的相互轉換

//Excel列字母轉數字
public static int ToIndex(string columnName)
{
    if (!Regex.IsMatch(columnName.ToUpper(), @"[A-Z]+")) { throw new Exception("invalid parameter"); }

    int index = 0;
    char[] chars = columnName.ToUpper().ToCharArray();
    for (int i = 0; i < chars.Length; i++)
    {
        index += ((int)chars[i] - (int)'A' + 1) * (int)Math.Pow(26, chars.Length - i - 1);
    }
    return index - 1;
}

//Excel數字轉列字母
public static string ToName(int index)
{
    if (index < 0) { throw new Exception("invalid parameter"); }

    List<string> chars = new List<string>();
    do
    {
        if (chars.Count > 0) index--;
        chars.Insert(0, ((char)(index % 26 + (int)'A')).ToString());
        index = (int)((index - index % 26) / 26);
    } while (index > 0);

    return String.Join(string.Empty, chars.ToArray());
}