Optional Interface in Class Libraries

Question

I have an interface called IProjectUser that defines a read function and a write function for reading and writing to project files. I also have a class called Project that holds a generic list of IProjectUser objects to manage project files. Both of these are in the class library Project.dll.

I also have a class library called A.dll that contains a class called Foo which implements IProjectUser. The ability to read/write project files is secondary to this class. It holds and manipulates some data. A.dll references Project.dll.

The application also contains some forms and other classes that implement IProjectUser.

I can imagine a situation in the future where I might want to use A.dll in another project that doesn't use project files. However I will be forced to include Project.dll just because A.dll requires it. Even though the functionality is optional.

Is there a different design pattern that would allow me to essentially make an interface optional?

I hope I explained this clearly enough.

Update

What about casting objects to interfaces? This would open up the possibility that an interface is not implemented correctly. Is that a good or bad design approach for this kind of problem?

if (Foo is IProjectUser) {
  ProjectUsers.Add(Foo as IProjectUser);
  // etc
}

Answer 1

Use inherited or multiple interfaces. You cannot make an interface method optional.

Answer 2

Casting your object to an interface that it doesn't implement will not work -- you'll end up with a null value in your variable. What's wrong with using proper design and adding B.dll as suggested below? A.dll becomes completely reusable, and you still get to have a version of Foo that implements IProjectUser.

1. Drop the reference to Project.dll from A.dll.
2. Drop IProjectUser from Foo.
3. Create B.dll which references Project.dll and A.dll.
4. Create FooProjectUser in B.dll which inherits from Foo and implements IProjectUser.
5. Move the project specific logic from Foo into FooProjectUser.
6. Use FooProjectUser in the places where you currently use Foo, freeing A.dll from any references to Project.dll.