linkbutton指向iframe

 
   protected void LinkButton1_Click(object sender, EventArgs e)
        {
            string Str = "select ClassId from Type_Class where ClassPre='200906191526456875'";
            DataSet ds = Tool.DataAccess.dataSet(Str);
            string ClassId = Server.UrlEncode(ds.Tables[0].Rows[0]["ClassId"].ToString().Trim());            ScriptManager.RegisterStartupScript(Page,
                this.GetType(),
                "updateScript",
                "document.getElementById('ContentPage').src='WebForm2.aspx?ID='" + ClassId, true);
            
            //Label1.Text = (Master.FindControl("Repeater1").FindControl("LinkButton1") as LinkButton).Text;
        }
ContentPage为iframe的id，页面跳转不过去怎么回事？