RequiredFieldValidator

@inherits ControlComponent

<table cellpadding="10" style="background-color:#eeeeee;">
                                            <tr valign="top">
                                            <td colspan="3">
                                            <asp.Label @ref="this.output"
                                            Text="Fill in the required field below" />
                                            <br />
                                            </td>
                                            </tr>
                                            <tr>
                                            <td colspan="3">
                                            <b>Credit Card Information</b>
                                            </td>
                                            </tr>
                                            <tr>
                                            <td align="right">
            Card Number:
                                            </td>
                                            <td>
                                            <asp.TextBox ID="TextBox1" />
                                            </td>
                                            <td>
                                            <asp.RequiredFieldValidator ControlToValidate="TextBox1"
                                            Display="ValidatorDisplay.Static"
                                            ErrorMessage="*" />
                                            </td>
                                            </tr>
                                            <tr>
                                            <td></td>
                                            <td colspan="2">
                                            <asp.Button Text="Validate"
                                            OnClick="this.ValidateBtn_Click" />
                                            </td>
                                            </tr>
</table>

@code {
                                            private Label output;

                                            protected void ValidateBtn_Click(object sender, EventArgs e)
    {
                                            if (this.Page.IsValid)
        {
            output.Text = "Required field is filled!";
        }
                                            else
        {
            output.Text = "Required field is empty!";
        }
    }
}