This is an odd change on a Microsoft "best practice..." For .NET Framework v1.x, it was recommended that you always inherit from ApplicationException when creating custom exception classes for your application.

But for Framework 2.x, they have revoked this recommendation, as you can see here...

For most applications, derive custom exceptions from the Exception class. It was originally thought that custom exceptions should derive from the ApplicationException class; however in practice this has not been found to add significant value.

I'm not sure why they made this distinction? ApplicationException is inherited from Exception anyway, and I can't imagine there is significant performance issues there? Anyone? Bueller?