Why is the use of interface references (e.g. IDataReader and IDbConnection) preferred over use of references whose type matches the database specific classes being created (e.g. SqlDataReader and SqlConnection)?

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The interfaces offer more services than the classes so the interface references are more powerful

The classes are all abstract so they can not be used directly

It makes the code more generic

It is more efficient to call a method through an interface reference

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Using the interface reference makes the code more generic by reducing the amount of database specific classes hardcoded into the application. It is therefore much easier to switch databases if needed.