private const string cByte = "BYTE";            // binary                        
        private const string cBoolean = "BOOLEAN";      // bit                           
        private const string cInt32 = "INT32";          // int                           
        private const string cInt64 = "INT64";          // bigint                           
        private const string cDecimal = "DECIMAL";      // numeric                       
        private const string cDouble = "DOUBLE";        // float
        private const string cDateTime = "DATETIME";    // datetime
        private const string cDate = "DATE";            // date
        private const string cGUID = "GUID";            // GUID

                        if (((DataColumn)t.Columns[j]).DataType.Name.ToUpper().Contains(cGUID))
                        {
                            workSheet.Cells[startRow + 1, counter, endIndex, counter].NumberFormat = "@";
                        }

                            if (((DataColumn)t.Columns[j]).DataType.Name.ToUpper().Contains(cGUID))
                            {
                                workSheet.Cells[j + startRow + 1, counter].Value = t.Rows[j][k].ToString().Trim().ToUpper();
                            }

                        else if (((DataColumn)t.Columns[j]).DataType == System.Type.GetType("System.Date"))
                        {
                            workSheet.Cells[startRow + 1, counter, endIndex, counter].NumberFormat = "DD.MM.YYYY";
                        }
                        else if (((DataColumn)t.Columns[j]).DataType == System.Type.GetType("System.DateTime"))
                        {
                            workSheet.Cells[startRow + 1, counter, endIndex, counter].NumberFormat = "DD.MM.YYYY h:mm:ss;@";
                        }