How to export selected gridview items to excel and save the excel workbook in folder in asp.net

so far i have tried with below code

protected void Button1_Click(object sender, EventArgs e)
{
DataTable dt = new DataTable();
dt.Columns.Add("Doc_id");
dt.Columns.Add("Document_Name");
dt.Columns.Add("Rcname");
dt.Columns.Add("PostedBy");
foreach (GridViewRow row in gvdownloaddocuments.Rows)
{
CheckBox chk = (CheckBox)row.FindControl("CheckBox1");
if (chk.Checked == true)
{
int i = row.RowIndex;
Label lbl = (Label)gvdownloaddocuments.Rows[i].FindControl("lbl_docid");
Label lbl1 = (Label)gvdownloaddocuments.Rows[i].FindControl("lblFileName");
Label lbl2 = (Label)gvdownloaddocuments.Rows[i].FindControl("lbl_rcname");
Label lbl3 = (Label)gvdownloaddocuments.Rows[i].FindControl("lbl_postedby");
DataRow dr = dt.NewRow();
dr["Doc_id"] = Convert.ToString(lbl.Text);
dr["Document_Name"] = Convert.ToString(lbl1.Text);
dr["Rcname"] = Convert.ToString(lbl2.Text);
dr["PostedBy"] = Convert.ToString(lbl3.Text);

dt.Rows.Add(dr);
}
}
GridView GridView1 = new GridView();
GridView1.DataSource = dt;
GridView1.DataBind();
Response.Clear();
Response.Buffer = true;
Response.ContentType = "application/ms-excel";
Response.AddHeader("content-disposition", string.Format("attachment;filename={0}.xls", "selectedrows"));
Response.Charset = "";
System.IO.StringWriter stringwriter = new StringWriter();
HtmlTextWriter htmlwriter = new HtmlTextWriter(stringwriter);
gvdownloaddocuments.RenderControl(htmlwriter);
// gvdownloaddocuments.Attributes.Add("runat=server");
Response.Write(stringwriter.ToString());
Response.End();


}

public override void VerifyRenderingInServerForm(Control control)
{
/*Verifies that the control is rendered */
}
--------
But m getting the following error
-----------------
RegisterForEventValidation can only be called during Render();