Qu'est-ce que the algorithm to convert an Excel Column Letter into its Number?

[Clippy]

I need an algorithm to convert an Excel Column letter to its proper number.

The language this will be written in is C#, but any would do or even pseudo code.

Please note I am going to put this in C# and I don’t want to use the office dll.

For ‘A’ the expected result will be 1

For ‘AH’ = 34

For ‘XFD’ = 16384

[ayi]

public static int ExcelColumnNameToNumber(string columnName)
{
    if (string.IsNullOrEmpty(columnName)) throw new ArgumentNullException("columnName");

    columnName = columnName.ToUpperInvariant();

    int sum = 0;

    for (int i = 0; i < columnName.Length; i++)
    {
        sum *= 26;
        sum += (columnName[i] - 'A' + 1);
    }

    return sum;
}