How to export GridView data to excel in asp.net in C#.

 protected void btnexport_Click(object sender, EventArgs e)
 {
        string attachment = "attachment;         filename=RepurchaseRpt".xls";
        Response.Clear();
        Response.AddHeader("content-disposition", attachment);
        Response.Charset = "";
        Response.ContentType = "application/vnd.xls";
        System.IO.StringWriter stringWrite = new System.IO.StringWriter();
        System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
        GridView1.RenderControl(htmlWrite);
        Response.Write(stringWrite.ToString());
        Response.End();
    }
    public override void VerifyRenderingInServerForm(Control control)
    {
    }