Important properties of ValidationSummary control : ValidationSummary « Validation « ASP.NET Tutorial

DisplayMode: how the error messages are formatted. 
             Possible values are BulletList, List, and SingleParagraph.

HeaderText:  display header text above the validation summary.

ShowMessageBox: display a popup alert box.

ShowSummary:    hide the validation summary in the page.

<%@ Page Language="C#" %>
<!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>Show Summary Popup</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>

    <asp:ValidationSummary
        id="ValidationSummary1"
        ShowMessageBox="true"
        ShowSummary="false"
        Runat="server" />

    <asp:Label
        id="lblFirstName"
        Text="First Name:"
        AssociatedControlID="txtFirstName"
        Runat="server" />
    <br />
    <asp:TextBox
        id="txtFirstName"
        Runat="server" />
    <asp:RequiredFieldValidator
        id="reqFirstName"
        ErrorMessage="First Name is required"
        ControlToValidate="txtFirstName"
        Display="None"
        Runat="server" />

    <br /><br />

    <asp:Label
        id="lblLastName"
        Text="Last Name:"
        AssociatedControlID="txtLastName"
        Runat="server" />
    <br />
    <asp:TextBox
        id="txtLastName"
        Runat="server" />
    <asp:RequiredFieldValidator
        id="reqLastName"
        ErrorMessage="Last Name is required"
        ControlToValidate="txtLastName"
        Display="None"
        Runat="server" />

    <br /><br />

    <asp:Button
        id="btnSubmit"
        Text="Submit"
        Runat="server" />

    </div>
    </form>
</body>
</html>

8.13.ValidationSummary
	8.13.1.	ValidationSummary displays a list of validation errors
	8.13.2.	Important properties of ValidationSummary control
	8.13.3.	Use ValidationSummary to display message box (C#)
	8.13.4.	RequiredFieldValidatorSummary DisplayMode
	8.13.5.	ValidationSummary DisplayMode=BulletList