.net 验证输入不为空

<tr>
                <td class="style4">
                    Re-Type New Password</td>
                <td align="left">
                    <asp:TextBox ID="txtNewPassword2" runat="server" TextMode="Password"></asp:TextBox>
                    <asp:RequiredFieldValidator runat="server" ID="RequiredFieldValidator4"
                                                        ControlToValidate="txtNewPassword2" ValidationGroup="ValidateGroup1" Display="Dynamic" ErrorMessage="New Password confirm is required." />
                </td>
            </tr>
            <tr>
                <td class="style4">
                    &nbsp;</td>
                <td align="left">
                    <asp:Button ID="btnSave" runat="server" Text="Save" OnClick="btnSave_Click" ValidationGroup="ValidateGroup1" />
                </td>
            </tr>