How to Correctly Model this Polymorphic Association?

Given the following Entities:

Container

User

Client

Institution

The Container entity has an association to one or more parties through the property AssignedToParties.

Container.AssignedToParties can contain a mix of Users, Clients, and Institutions.

What is the recommened domain model for this relationship.

I had considered the following options:

1) Create separate properties for each type:

Container.AssignedToUsers

Container.AssignedToClients

Container.AssignedToInstitutions

This seems pretty inelegant, but does not require business logic to check the types or do any downcasting.

2) Create a common base class "Party" for User/Client/Institution

Container.AssignedToParties would then be a collection of Party entities. This seems like an akward solution since the Party base class wouldn't have any methods or properties. I'm also not sure I like the idea of adding 开发者_开发问答one more layer of inheritance here.

This solution, like #3, would require the system to check the types at runtime to make decisions and then downcast to either User/Client/Institution to process them.

3) Create a Marker Interface IContainerAssignable that User/Client/Institution implement

This would at least provide some type safety, but would require type checking and downcasting.

Right now, I'm leaning towards #3. It seems the simplest, but I've read in quit a few places that if your code is running logic that has to test for a given type and downcast that you probably have a bad design.

Any advice appreciated.

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Based on your comment in response to my question, I would not have the assignment handled by the container at all. To use your example, a file doesn't know or care who's watching it.

Instead, I'd either have the watcher implement some method (or collection property) to start watching a container (and in that case, making them an IContainerWatcher or the like would make sense), or else have that watching functionality offloaded entirely into a service that's dedicated to maintaining the associations, like a publish/subscribe mechanism does. This is conceptually analogous to a many-to-many join table in a database schema.