Export Gridview to Excel not working

Hello, I have the following code, I put this into App_code.

 

public static void PrepareGridViewForExport(Control gv)
    {

        LinkButton lb = new LinkButton();
        Literal l = new Literal();
        string name = String.Empty;
        for (int i = 0; i < gv.Controls.Count; i++)        {

            if (gv.Controls[i].GetType() == typeof(LinkButton))
            {
                l.Text = (gv.Controls[i] as LinkButton).Text;
                gv.Controls.Remove(gv.Controls[i]);
                gv.Controls.AddAt(i, l);

            }
            else if (gv.Controls[i].GetType() == typeof(DropDownList))
            {
                l.Text = (gv.Controls[i] as DropDownList).SelectedItem.Text;
                gv.Controls.Remove(gv.Controls[i]);
                gv.Controls.AddAt(i, l);
            }
            else if (gv.Controls[i].GetType() == typeof(CheckBox))
            {
                l.Text = (gv.Controls[i] as CheckBox).Checked ? "True" : "False";
                gv.Controls.Remove(gv.Controls[i]);
                gv.Controls.AddAt(i, l);
            }
            if (gv.Controls[i].HasControls())
            {
                PrepareGridViewForExport(gv.Controls[i]);
            }

        }

    }

    public static void ExportGridView(GridView g1)
    {
        string attachment = "attachment; filename=Attachment.xls";
        HttpContext.Current.Response.ClearContent();
        HttpContext.Current.Response.AddHeader("content-disposition", attachment);
        HttpContext.Current.Response.ContentType = "application/ms-excel";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        g1.RenderControl(htw);
        HttpContext.Current.Response.Write(sw.ToString());
        HttpContext.Current.Response.End();

    }

 

 

But I got this error

System.Web.HttpException was unhandled by user code
  Message="Control 'ctl00_ContentPlaceHolder1_GridView1' of type 'GridView' must be placed inside a form tag with runat=server."
  Source="System.Web"
  ErrorCode=-2147467259
  StackTrace:
       at System.Web.UI.Page.VerifyRenderingInServerForm(Control control)
       at System.Web.UI.WebControls.GridView.Render(HtmlTextWriter writer, Boolean renderPanel)
       at System.Web.UI.WebControls.GridView.Render(HtmlTextWriter writer)
       at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
       at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
       at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
       at ExcelHelper.ExportGridView(GridView g1) in c:\Inetpub\wwwroot\WebSite\App_Code\ExcelHelper.cs:line 67
       at ListDetails.Button3_Click(Object sender, EventArgs e) in c:\Inetpub\wwwroot\WebSite\DynamicData\CustomPages\Cities\ListDetails.aspx.cs:line 110
       at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
       at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
       at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
       at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
       at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  InnerException: View the Solution FREE for 7 Days