Ext.NET with ASP.NET Validation

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">
    protected override void OnLoad(EventArgs e) {
        base.OnLoad(e);

        Page.Validate();

    }

    void ServerValidation(object source, ServerValidateEventArgs args)
    {
        try
        {
            // Test whether the value entered into the text box is even.
            int i = int.Parse(args.Value);
            args.IsValid = ((i % 2) == 0);
        }
        catch (Exception ex)
        {
            args.IsValid = false;
        }
    }

    void Button_Click(object sender, EventArgs e) {
        // Display whether the page passed validation.
        if (Page.IsValid) {
            Label1.Text = "Page is valid.";
        } else {
            Label1.Text = "Page is not valid!";
        }
    }


</script>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>Ext.NET Example</title>
</head>
<body>
    <form id="Form1" runat="server">
    <ext:ResourceManager ID="ResourceManager1" runat="server" />
    <ext:Label ID="Label1" runat="server" Text="Enter an even number:" />
    <br/>
    <ext:TextField ID="TextField1" runat="server" />
    <asp:CustomValidator ID="CustomValidator1"
        runat="server"
        ControlToValidate="TextField1"
        OnServerValidate="ServerValidation"
        ErrorMessage="Not an even number!" />
    <ext:Button ID="Button1" runat="server" Text="Validate" AutoPostBack="false" CausesValidation="true">
        <DirectEvents>
            <Click OnEvent="Button_Click" />
        </DirectEvents>
    </ext:Button>
    <br/>
    </form>
</body>
</html>