textbox 增加click事件

<asp:TextBox ID="tb" runat="server" SkinID="forFormSingleLine" ReadOnly="true"></asp:TextBox>
<asp:Button ID="btn" runat="server" Text="Button" onclick="btn_Click" Visible="false" />

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                tb.Attributes["onclick"] = "window.showModalDialog(\"http://www.cnblogs.com/PublicInfo/FrmContractListDlg.aspx"
              + " \",\" \",\"status:false;dialogWidth:800px;dialogHeight:530px\");";
                tb.Attributes["onclick"] += ClientScript.GetPostBackEventReference(btn, null);
            }
        }


       
        protected void btn_Click(object sender, EventArgs e)
        {
　　　　　　　　//to do smth...
　　　　　 }