export windows form datagridview to excel file using c#

1- add reference to the library
Microsoft.Office.Interop.Excel
2- copy past the following funciton to export excel sheet (it saved in temp directory for current user )

System.Threading.Thread.CurrentThread.CurrentCulture = System.Globalization.CultureInfo.CreateSpecificCulture("en-US");
            // creating Excel Application
            Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();


            // creating new WorkBook within Excel application
            Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);


            // creating new Excelsheet in workbook
            Microsoft.Office.Interop.Excel._Worksheet worksheet = null;

            // see the excel sheet behind the program
            app.Visible = true;

            // get the reference of first sheet. By default its name is Sheet1.
            // store its reference to worksheet
            worksheet =(Microsoft.Office.Interop.Excel._Worksheet)workbook.Sheets["Sheet1"];
            worksheet = (Microsoft.Office.Interop.Excel._Worksheet)workbook.ActiveSheet;

            // changing the name of active sheet
            worksheet.Name = "Exported from gridview";


            // storing header part in Excel
            for (int i = 1; i < grdCity.Columns.Count + 1; i++)
            {
                worksheet.Cells[1, i] = grdCity.Columns[i - 1].HeaderText;
            }



            // storing Each row and column value to excel sheet
            for (int i = 0; i < grdCity.Rows.Count - 1; i++)
            {
                for (int j = 0; j < grdCity.Columns.Count-1; j++)
                {
                    worksheet.Cells[i + 2, j + 1] = grdCity.Rows[i].Cells[j].Value.ToString();
                }
            }

            string myTempFile = Path.Combine(Path.GetTempPath(), "ratings.xlsx");
            // save the application
            workbook.SaveCopyAs(myTempFile);// (myTempFile, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            worksheet = null;
            workbook = null;
           // File.Open(myTempFile, FileMode.Open);
            // Exit from the application
            app.Quit();

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Export DataTable To Excel

 public static  void ExporttoExcel(DataTable table)
     {
         var sb = new StringBuilder();
         HttpContext.Current.Response.Clear();
         HttpContext.Current.Response.ClearContent();
         HttpContext.Current.Response.ClearHeaders();
         HttpContext.Current.Response.Buffer = true;
         HttpContext.Current.Response.ContentType = "application/ms-excel;charset=windows-1256";
         sb.Append(@"<!DOCTYPE HTML PUBLIC ""-//W3C//DTD HTML 4.0 Transitional//EN"">");
         HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment;filename=Reports.xls");
         HttpContext.Current.Response.Charset = "UTF-8";
         HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.UTF8;
         sb.Append("<font style='font-size:10.0pt; font-family:Calibri;'>");
         sb.Append("<BR><BR><BR>");
         //sets the table border, cell spacing, border color, font of the text, background, foreground, font height
         sb.Append("<Table border='1' bgColor='#ffffff' " +
           "borderColor='#000000' cellSpacing='0' cellPadding='0' " +
           "style='font-size:10.0pt; font-family:Calibri; background:white;'> <TR>");
         //am getting my grid's column headers
         int columnscount = table.Columns.Count;

         for (int j = 0; j < columnscount; j++)
         {      //write in new column
             sb.Append("<Td>");
             //Get column headers  and make it as bold in excel columns
             sb.Append("<B>");
             sb.Append(table.Columns[j].ColumnName.ToString());
             sb.Append("</B>");
             sb.Append("</Td>");
         }
         sb.Append("</TR>");
         foreach (DataRow row in table.Rows)
         {//write in new row
             sb.Append("<TR>");
             for (int i = 0; i < table.Columns.Count; i++)
             {
                 sb.Append("<Td>");
                 sb.Append(row[i].ToString());
                 sb.Append("</Td>");
             }

             sb.Append("</TR>");
         }
         sb.Append("</Table>");
         sb.Append("</font>");
         HttpContext.Current.Response.Write(sb);
         HttpContext.Current.Response.Flush();
         HttpContext.Current.Response.End();
     }