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).