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…

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:

 First Name  Last Name  Date of Birth  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?