Fastest function to generate Excel column letters in C#

Question

What is the fastest c# function that takes and int and returns a string containing a letter or letters for use in an Excel function? For example, 1 returns "A", 26 returns "Z", 27 returns "AA", etc.

This is called tens of thousands of times and is taking 25% of the time needed to generate a large spreadsheet with many formulas.

public string Letter(int intCol) {

    int intFirstLetter = ((intCol) / 676) + 64;
    int intSecondLetter = ((intCol % 676) / 26) + 64;
    int intThirdLetter = (intCol % 26) + 65;

    char FirstLetter = (intFirstLetter > 64) ? (char)intFirstLetter : ' ';
    char SecondLetter = (intSecondLetter > 64) ? (char)intSecondLetter : ' ';
    char ThirdLetter = (char)intThirdLetter;

    return string.Concat(FirstLetter, SecondLetter, ThirdLetter).Trim();
}

Answer 1

I currently use this, with Excel 2007

public static string ExcelColumnFromNumber(int column)
        {
            string columnString = "";
            decimal columnNumber = column;
            while (columnNumber > 0)
            {
                decimal currentLetterNumber = (columnNumber - 1) % 26;
                char currentLetter = (char)(currentLetterNumber + 65);
                columnString = currentLetter + columnString;
                columnNumber = (columnNumber - (currentLetterNumber + 1)) / 26;
            }
            return columnString;
        }

and

public static int NumberFromExcelColumn(string column)
        {
            int retVal = 0;
            string col = column.ToUpper();
            for (int iChar = col.Length - 1; iChar >= 0; iChar--)
            {
                char colPiece = col[iChar];
                int colNum = colPiece - 64;
                retVal = retVal + colNum * (int)Math.Pow(26, col.Length - (iChar + 1));
            }
            return retVal;
        }

As mentioned in other posts, the results can be cached.