Base26 Number Encoding/Generate Excel Columns

Ever needed to convert an Excel Column Letter to a number or vice versa?  Here’s a couple of extension methods to do it:

1: public static string ToLetterEncoded(this Int32 numberToEncode)
2: {
3:     if (numberToEncode <= 0) throw new ArgumentOutOfRangeException(
           "numberToEncode", numberToEncode, "Value must be greater than zero");
4:     numberToEncode--;
5:     if (numberToEncode >= 0 && numberToEncode < 26)
6:         return ((char)('A' + numberToEncode)).ToString();
7:     else
8:         return ToLetterEncoded(numberToEncode / 26) +
               ToLetterEncoded(numberToEncode % 26 + 1);
9: }

  1: public static Int32 FromLetterEncodedInt32(this string letterEncodedNumber)
  2: {
  3:     if (letterEncodedNumber.IsNullOrWhiteSpace()) throw new ArgumentNullException(
             "letterEncodedNumber");
         //smallest column first
  4:     char[] letters =
             letterEncodedNumber.ToUpperInvariant().ToCharArray().Reverse().ToArray();
  5:     if (letters.Where(c => !char.IsLetter(c)).Any()) throw new ArgumentOutOfRangeException(
             "letterEncodedNumber",
             "Encoded Number must only contain the letters A-Z");
  6:     int[] numbers = letters.Select(c => (((int)c - 'A') + 1)).ToArray();
  7:     Int32 columnNumber = 0;
  8:     for (int i = 0; i < letters.Length; i++)
  9:     {
 10:         columnNumber += (Int32)Math.Pow(26, i) * numbers[i];
 11:     }
 12:     return columnNumber;
13: }

I’ve used Int32 to represent the type of integer that you’re en(de)coding, for easy find ‘n’ replace.

These will only work for positive integer (1 and over).