RequiredFieldValidator验证控件的应用

<form id="form1" runat="server">
    <div>
    <center>
        <table border="1" cellpadding="0" cellspacing="0">
            <tr>
                <td colspan="2" style="font-size:32pt; color: White; background-color: #669900">
                    留言板
                </td>
            </tr>
            <tr>
                <td style="font-size: 10pt">
                用户名：
                </td>
                <td style="width: 4px">
                    <asp:TextBox ID="UserNameTextBox" runat="server" Enabled="false"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td style="font-size: 10pt">
                留言主题：
                </td>
                <td align="left">
                    <asp:TextBox ID="themeTextBox" runat="server" Width="233px"></asp:TextBox>
                    <asp:RequiredFieldValidator ID="themeRequiredFieldValidator" runat="server" 
                    ErrorMessage="留言主题不能为空" ControlToValidate="themeTextBox">*</asp:RequiredFieldValidator>
                </td>
            </tr>
            <tr>
               <td style="font-size: 10pt">
                留言内容：
                </td>
                <td align="left">
                    <asp:TextBox ID="contentTextBox" runat="server" Height="214px" Width="233px"></asp:TextBox>
                    <asp:RequiredFieldValidator ID="contentRequiredFieldValidator" runat="server" 
                    ErrorMessage="留言内容不能为空" ControlToValidate="contentTextBox">*</asp:RequiredFieldValidator>
                </td>
            </tr>
            <tr>
                <td colspan="2">
                    <asp:Button ID="messageButton" runat="server" Text="留言" />&nbsp &nbsp
                    <asp:Button ID="cancelButton" runat="server" Text="取消" />
                </td>
            </tr>
            <tr>
                <td colspan="2" style="color: Black">
                    <span style="font-size: smaller">本站共有</span>
                    <asp:Label ID="Label1" runat="server" ForeColor="#C00000" Width="2px"></asp:Label>
                     <span style="font-size: smaller">位注册会员</span>
                </td>
            </tr>
        </table>
    </center>
    </div>
      <asp:validationsummary runat="server" ID="validationsummary1" ShowMessageBox="true" ShowSummary="false"></asp:validationsummary>
    </form>