CustomValidator for login page : CustomValidator « Validation « ASP.NET Tutorial

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    <title>CustomValidator Control Sample</title>
    <script runat="server" language="C#">
      void CheckUniqueUserName(Object s, ServerValidateEventArgs e)
        string username = e.Value.ToLower();
        if(username == "user" || username == "password")
          e.IsValid = false;
      void submitButton_Click(Object s, EventArgs e)
          submitButton.Text = "Valid";
          submitButton.Text = "Invalid!";
    <form runat="server">
        New Username:<br />
        <asp:TextBox ID="usernameTextBox" runat="server" />
        <asp:CustomValidator ID="usernameUnique" runat="server"
            ErrorMessage="This username already taken!" />
        <asp:Button ID="submitButton" runat="server"
            OnClick="submitButton_Click" Text="Submit" />

8.3.1.You can associate a custom validation function with the CustomValidator control.
8.3.2.Validate a blank field
8.3.3.Performing validation against no particular field.
8.3.4.Use both client side and server side script to validate (
8.3.5.Create user control based on CustomValidator
8.3.6.Using the CustomValidator control to perform client-side validations
8.3.7.Using the CustomValidator control to perform server-side validations (C#)
8.3.8.Using the CustomValidator control to perform server-side validations (VB)
8.3.9.Custom validator with Javascript
8.3.10.CustomValidator for login page
8.3.11.CustomValidator for login page (VB)