Is it OK to use manager classes as Singletons?

For each concrete class I have a Manager class. This class has methods like GetAll(), GetById(), Save, etc.

I've made these manager classes a Singleton because I always need one instance, and I have the ability to cache results. For example, when I call GetAll() and the next time I need this method again, the manager don't have to go to the database, it can return the cached results.

Is this a good approach? Or is there a better a开发者_如何转开发lternative way?

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What you call manager classes are really "repositories"

Repositories should only work at an aggregate root level, not one repository per class. E.g. if I have an Order class which has a collection of OrderItem, then an Order repository would exist that would have Get/GetAll methods - as the Order is the agg root in this case.

All repository classes would usually be singleton classes, where you can usually enforce this easily via an IOC container.

Overall, I would say your approach of one "repository" per entity is bad - stick to one repository per aggregate root instead.

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Why not include them as part of the concrete class but static? Saves the need for two seperate classes.

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It sounds like you are close to implementing the Repository pattern, but not quite all the way there. I'd suggest looking into it. I wouldn't make them Singletons -- it makes it too hard to mock them out for your unit tests so you end up adding back doors to defeat the Singleton for testing. A cache makes a nice Singleton object, but why not simply share the cache this way instead of multiplying Singletons?

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For testing/mocking purposes, I would advise against using a Manager/Repository Singleton.

If you do want to cache results, then I would suggest delegating to a dedicated Cache class. Any static magic can be contained within the Cache class and your Manager/Repository's semantics can be kept clean.

From a Single Responsibility Principle point of view, I should be able to understand how a Manager/Repository works without having to understand your caching scheme.