You are creating a Windows Presentation Foundation application by using Microsoft .NET Framework 3.5.
The application includes the following features:
* A data provider control.
* Twenty TextBox controls bound to the data provider.
* A Submit button that saves the TextBox values to a data source.
Business rules state that the value entered in the UserName TextBox control cannot be the same as any other UserName values in the data source.
You need to provide feedback to the user as early as possible when a duplicate value is entered in the UserName TextBox control.
What should you do?

A.
Use the ExceptionValidationRule class.

B.
Use the OneWay BindingMode enumeration value.

C.
Create and use a class that inherits from the ValidationRule base class.

D.
Add an exception handler to the Click event handler for the Submit button.