Export data from Gridview to Excel

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C#3.5

MS-Excel

Hi all,
I am using the below code to export data from Gridview to Excel. But the problem I am getting is that my data in excel is coming as an image of Gridview and not in excel format. Can anyone suggest me the reason?

protected void btnexporttoxl_Click(object sender, EventArgs e)
{       
        Response.Clear();
        Response.AddHeader("content-disposition", "attachment;filename=FileName.xls");
        Response.Charset = "";
        // If you want the option to open the Excel file without saving than
        // comment out the line below
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        Response.ContentType = "application/vnd.xls";
        System.IO.StringWriter stringWrite = new System.IO.StringWriter();
        System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);        
        gd1.RenderControl(htmlWrite);
        Response.Write(stringWrite.ToString());
        Response.End();
}
public override void VerifyRenderingInServerForm(Control control)
{
        // Confirms that an HtmlForm control is rendered for the specified ASP.NET server control at run time.
        return;
}