Server cannot append header after HTTP headers have been sent.

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C#

ASP.NET

VS2010

Hi, I gotten this error " Server cannot append header after HTTP headers have been sent."

I Don't know where this is caused by in my code, anyone able to spot the error . Your help is appreciated, thank you !

private void ExportExcel()
  {
      HttpContext.Current.Response.Clear();
      HttpContext.Current.Response.Buffer = true;
      HttpContext.Current.Response.AddHeader("content-disposition", "attachment;filename=GridViewExport.xls");
      HttpContext.Current.Response.Charset = "";
      HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";
      using (StringWriter sw = new StringWriter())
      {
          HtmlTextWriter hw = new HtmlTextWriter(sw);
          //To Export all pages
          GridView1.AllowPaging = false;
          this.bindGridview1();
          GridView1.HeaderRow.BackColor = Color.White;
          foreach (TableCell cell in GridView1.HeaderRow.Cells)
          {
              cell.BackColor = GridView1.HeaderStyle.BackColor;
          }
          foreach (GridViewRow row in GridView1.Rows)
          {
              row.BackColor = Color.White;
              foreach (TableCell cell in row.Cells)
              {
                  if (row.RowIndex % 2 == 0)
                  {
                      cell.BackColor = GridView1.AlternatingRowStyle.BackColor;
                  }
                  else
                  {
                      cell.BackColor = GridView1.RowStyle.BackColor;
                  }
                  cell.CssClass = "textmode";
              }
          }
          GridView1.RenderControl(hw);
          //style to format numbers to string
          string style = @"<style> .textmode { mso-number-format:\@; } </style>";
          HttpContext.Current.Response.Write(style);
          HttpContext.Current.Response.Output.Write(sw.ToString());
          HttpContext.Current.Response.Flush();
          HttpContext.Current.Response.SuppressContent = true;
          HttpContext.Current.ApplicationInstance.CompleteRequest();
      }

  }

protected void btnDel_Click(object sender, EventArgs e)
    {
        try
        {
            PanelPopup2.Visible = true;
            while (PanelPopup2.Visible == true)
            ExportExcel();
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message);
        }
    }