excel range address by numbers

If I understand you correctly, this is how I do it is C#

        private static CellCoordinates ParseA1Notation(string cellAddr)
        {
            CellCoordinates returnValue = new CellCoordinates();
            try
            {
                Dictionary<string, int> row_col = new Dictionary<string, int>();
                int ii = 0, jj, colVal = 0;
                while (cellAddr[ii++] >= 'A') { };
                ii--;

                for (jj = 0; jj < ii; jj++) colVal = 26 * colVal + cellAddr[jj] - 'A' + 1;
                returnValue.x = colVal;
                returnValue.y = Int32.Parse(cellAddr.Substring(ii));
            }
            catch (Exception ex)
            {
                logger.Error($"Error \"{ex.Message}\" parsing \"{cellAddr}\"");
            }
            return returnValue;
        }

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Thank you Shaun.  can you tell which librarys do I need to add ?

Probably just System.Collections.Generic and CellCoordinates is just a class

        private class CellCoordinates
        {
            public int x;
            public int y;
        }

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trying to convert to vb.net im getting an error "operator - is not defined for types char and char"
on    this:
             colVal = 26 * colVal + (cellAddr(jj) - "A"c) + 1