private void ExportToExl(bool firstTime)
        {
           //string path = string.Empty;               
            //Delete the file if it exists. 
            if (firstTime && File.Exists(savingFileName))
                File.Delete(savingFileName);
            if (firstTime)
            {
                //This is the first time of creating the excel file and the first sheet.
                // Create a spreadsheet document by supplying the filepath.
                // By default, AutoSave = true, Editable = true, and Type = xlsx.
                SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.
                    Create(savingFileName, SpreadsheetDocumentType.Workbook);
                // Add a WorkbookPart to the document.
                WorkbookPart workbookpart = spreadsheetDocument.AddWorkbookPart();
                workbookpart.Workbook = new DocumentFormat.OpenXml.Spreadsheet.Workbook();
                // Add a WorksheetPart to the WorkbookPart.
                var worksheetPart = workbookpart.AddNewPart<WorksheetPart>(); 
                var sheetData = new SheetData();
                worksheetPart.Worksheet = new DocumentFormat.OpenXml.Spreadsheet.Worksheet(sheetData);
                var bold1 = new System.Windows.Documents.Bold();
                DocumentFormat.OpenXml.Spreadsheet.CellFormat cf = new DocumentFormat.OpenXml.Spreadsheet.CellFormat();
                // Add Sheets to the Workbook.
                DocumentFormat.OpenXml.Spreadsheet.Sheets sheets;
                sheets = spreadsheetDocument.WorkbookPart.Workbook.
                    AppendChild<DocumentFormat.OpenXml.Spreadsheet.Sheets>(new DocumentFormat.OpenXml.Spreadsheet.Sheets());
                // Append a new worksheet and associate it with the workbook.
                var sheet = new Sheet()
                {
                    Id = spreadsheetDocument.WorkbookPart.
                        GetIdOfPart(worksheetPart),
                    SheetId = sheetId,
                    Name = "Sheet" + sheetId
                };
                sheets.Append(sheet);
                //Add Header Row.
                var headerRow = new Row();
                foreach (DataColumn column in ResultsData.Columns)
                {
                    var cell = new Cell { DataType = CellValues.String, CellValue = new CellValue(column.ColumnName) };
                    headerRow.AppendChild(cell);
                }
                sheetData.AppendChild(headerRow);
                foreach (DataRow row in ResultsData.Rows)
                {
                    var newRow = new Row();
                    foreach (DataColumn col in ResultsData.Columns)
                    {
                        var cell = new Cell
                        {
                            DataType = CellValues.String,
                            CellValue = new CellValue(row[col].ToString())
                        };
                        newRow.AppendChild(cell);
                    }
                    sheetData.AppendChild(newRow);
                }               
                workbookpart.Workbook.Save();
                spreadsheetDocument.Close();
            }
    }
Datatable is ResultsData