Persistence Ignorance

Persistence ignorance (PI) refers to types that need to be persisted, but whose code is unaffected by the choice of persistence technology. Such types in .NET are sometimes referred to as Plain Old CLR Objects (POCOs), because they do not need to inherit from a particular base class or implement a particular interface. Persistence ignorance is valuable because it allows the same business model to be persisted in multiple ways, offering additional flexibility to the application. Persistence choices might change over time, from one database technology to another, or additional forms of persistence might be required in addition to whatever the application started with (for example, using a Redis cache or Azure Cosmos DB in addition to a relational database).

Some examples of violations of this principle include:

• A required base class
• A required interface implementation
• Classes responsible for saving themselves (such as the Active Record pattern)
• Required default constructor
• Properties requiring virtual keyword
• Properties forced to use certain types (for example, collection properties must expose ICollection, not just IEnumerable)
• Persistence-specific required attributes

The requirement that classes have any of the above features or behaviors adds coupling between the types to be persisted and the choice of persistence technology, making it more difficult to adopt new data access strategies in the future.