What factors should be taken into consideration when writing a custom exception class?

Question

When are custom Exception classes most-valuable?
Are there cases when they should or should not be used? What are the benefits?

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Answer 1

My main reason for using custom exceptions would be encapsulation with multiple providers: having a SqlException leak out of a SqlDataAccess layer, and a SocketException out of a NetworkDataAccess layer makes calling code dependent on the particulars of your implementation. Better to wrap them into a DataAccessException or something.