how to print gridview data when i click a button?

i got error my code past below
protected void Page_Load(object sender, EventArgs e)
{
SqlConnection myConnection = new SqlConnection("Data Source=HIMANSHU;Initial Catalog=try;Trusted_Connection=yes;");
SqlDataAdapter Sqlad = new SqlDataAdapter("SELECT * FROM gri", myConnection);
DataSet dsSupplier = new DataSet();
Sqlad.Fill(dsSupplier);
GridView1.DataBind();
}
protected void Button1_Click(object sender, EventArgs e)
{
Response.Clear();
Response.AddHeader("content-disposition", "attachment; filename=SupplierList.xls");
Response.ContentType = "application/vnd.xls";
System.IO.StringWriter WriteItem = new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter htmlText = new HtmlTextWriter(WriteItem);
GridView1.RenderControl(htmlText);
Response.Write(WriteItem.ToString());
Response.End();
}
i got below error
Control 'GridView1' of type 'GridView' must be placed inside a form tag with runat=server.
plz help me

Here step by step to print any control visit this link

Also you can use this code sample gridview data export in pdf then you can print it show this link

http://www.netomatix.com/development/gridviewprinting.aspx