...
    foreach (DataRow row in data.Rows)
            {
                IRow dataRow = mainSheet.CreateRow(rowIndex);

                foreach (DataColumn column in data.Columns)
                {
                    ICell cell = dataRow.CreateCell(column.Ordinal);

                    if (row[column] is string)
                    {
                        cell.SetCellValue((string)row[column]);
                        //cell.SetCellType(CellType.STRING);
                    }
                    else if (row[column] is uint)
                    {
                        cell.SetCellValue((uint)row[column]);
                       // cell.SetCellType(CellType.NUMERIC);
                    }
                    else if (row[column] is int)
                    {
                        cell.SetCellValue((int)row[column]);
                        //cell.SetCellType(CellType.NUMERIC);
                    }

                    cell.CellStyle = cellStyle;
                }

                rowIndex++;
            }
...