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Quick question: Would it be a good or a bad idea to implement my domain-driven design style repositories as singletons? Why?
Or should I maybe use a dependency injector container to manage my repositories and decide if they are singletons or not?
I'm still reading DDD Quickly, and would like to see some good repository examples.
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Don't use static or singleton repositories because of: It affects testablility, you can not mock it when unit testing. It affects extensibility, you can not make more than one concrete implementation and you can not replace behavior without re-compiling.
The Repository Pattern has gained quite a bit of popularity since it was first introduced as a part of Domain-Driven Design in 2004. Essentially, it provides an abstraction of data, so that your application can work with a simple abstraction that has an interface approximating that of a collection.
Use your dependency injection container to decide how and where repositories are created.
By using
UserRepository.Instance.Find(userId);
you're creating a barrier to testing.
If your repositories are passed into services using Constructor Injection, then you can also easily replace them with mocks.
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