RequiredFieldValidator requires a user to enter a value into a form field before submitting the form. : RequiredFieldValidator « Validation « ASP.NET Tutorial

Two important properties when using the RequiredFieldValdiator control:

ControlToValidate: The ID of the form field being validated.
Text:              The error message.

<%@ 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 RequiredFieldValidator</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>

    <asp:Label
        id="lblFirstName"
        Text="First Name:"
        AssociatedControlID="txtFirstName"
        Runat="server" />
    <asp:TextBox
        id="txtFirstName"
        Runat="server" />
    <asp:RequiredFieldValidator
        id="reqFirstName"
        ControlToValidate="txtFirstName"
        Text="(Required)"
        Runat="server" />

    <asp:Label
        id="lblLastName"
        Text="Last Name:"
        AssociatedControlID="txtLastName"
        Runat="server" />
    <br />
    <asp:TextBox
        id="txtLastName"
        Runat="server" />
    <asp:RequiredFieldValidator
        id="reqLastName"
        ControlToValidate="txtLastName"
        Text="(Required)"
        Runat="server" />
    <asp:Button
        id="btnSubmit"
        Text="Submit"
        Runat="server" />

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

8.11.RequiredFieldValidator
	8.11.1.	RequiredFieldValidator requires a user to enter a value into a form field before submitting the form.
	8.11.2.	Use asp:RequiredFieldValidator to check the input of an asp:TextBox (VB.net)
	8.11.3.	Use the RequiredFieldValidator's InitialValue to set a default value
	8.11.4.	Specify ErrorMessage for asp:RequiredFieldValidator
	8.11.5.	asp:RequiredFieldValidator will be converted to JavaScript Code after loading in client side
	8.11.6.	A simple use of the RequiredFieldValidator server control (C#)
	8.11.7.	Text field of RequiredFieldValidator server control (VB)
	8.11.8.	Placing values between nodes
	8.11.9.	Working with the InitialValue property
	8.11.10.	Using two RequiredFieldValidator controls for one form element
	8.11.11.	Validating Drop-Down Lists with the RequiredFieldValidator Control
	8.11.12.	Using images for error notifications
	8.11.13.	Disabling client-side validations in a validation control
	8.11.14.	Using sound for error notifications
	8.11.15.	EnableClientScript=false, Display=Static
	8.11.16.	RequiredFieldValidator for login page
	8.11.17.	RequiredFieldValidator with server side validation
	8.11.18.	Hard code error message
	8.11.19.	RequiredFieldValidator Dynamic Display