Allen works as a Software Developer for ABC Inc. He develops an application using Visual Studio .NET

2005. The application will be used for the registration of employees by filling a form provided in the application. The form contains the following fields:

l First Name

l Last Name

l Date of Birth

l E-mail

He provides a TextBox control for each field. He wants an employee to be able to receive a user-defined error message when he makes a wrong entry. Which of the following properties is mandatory if a

CustomValidator control is used to validate an entry?