DropDownList in gridview binding at runtime

<asp:TemplateField>
            <HeaderTemplate>Emp</HeaderTemplate>
            <ItemTemplate ><asp:DropDownList ID="ddl1" runat ="server"   OnLoad ="ddl1_load"></asp:DropDownList></ItemTemplate>
</asp:TemplateField>

protected void ddl1_load(object sender, EventArgs e)
        {
            SqlDataAdapter da = new SqlDataAdapter("select sno from student", cn);
            DataTable dt = new DataTable();
            da.Fill(dt);
            DropDownList ddl = (DropDownList)sender;
            ddl.DataSource = dt;
            ddl.DataTextField = "sno";
            ddl.DataValueField = "sno";
            ddl.DataBind();
        }