Export Gridview to Excel

 protected void btnExtractExcel_Click(object sender, EventArgs e)  
     {  
       string attachment = string.Empty;  
       try  
       {  
         if (cID == "OLR")  
         {  
           attachment = "attachment; filename=ExtractOLRExcel" + System.DateTime.Now.ToString() + ".xls";  
         }  
         else if (cID == "BIT")  
         {  
           attachment = "attachment; filename=ExtractBITExcel" + System.DateTime.Now.ToString() + ".xls";  
         }  
         else if (cID == "FCU")  
         {  
           attachment = "attachment; filename=ExtractFCUExcel" + System.DateTime.Now.ToString() + ".xls";  
         }  
         else if (cID == "FCC")  
         {  
           attachment = "attachment; filename=ExtractFCUExcel-BFLHR" + System.DateTime.Now.ToString() + ".xls";  
         }  
         else if (cID == "FCUC")  
         {  
           attachment = "attachment; filename=ExtractFCUCompletedExcel" + System.DateTime.Now.ToString() + ".xls";  
         }  
         Response.ClearContent();  
         Response.AddHeader("content-disposition", attachment);  
         Response.ContentType = "application/vnd.ms-excel";  
         StringWriter sw = new StringWriter();  
         HtmlTextWriter htw = new HtmlTextWriter(sw);  
         // Create a form to contain the grid  
         HtmlForm frm = new HtmlForm();  
         summaryGrid.Parent.Controls.Add(frm);  
         frm.Controls.Add(summaryGrid);  
         frm.RenderControl(htw);  
         Response.Write(sw.ToString());  
         Response.End();  
       }  
       catch (Exception ex)  
       {  
         SPDiagnosticsService.Local.WriteTrace(0, new SPDiagnosticsCategory("BFL Delta Force-Recruitment-btnExtractExcel_Click", TraceSeverity.Unexpected, EventSeverity.Error), TraceSeverity.Unexpected, ex.Message + ex.StackTrace, null);  
       }  
     }