The Unit of Work pattern is designed to maintain a list of business objects that have been changed by a business transaction, whether by adding, removing, or updating. The Unit of Work then coordinates the persistence of the changes and any concurrency problems flagged. The benefit of utilizing the Unit of Work in your DAL is to ensure data integrity; if an issue arises partway through persisting a series of business objects as part of a transaction, all changes should be rolled back to ensure that the data remains in a valid state.
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