当该子项作为其父项存储在列表中时，如何访问Child类的接口属性？

Question

I think the title of this post might not explain my issue very well, so here is some code that should hopefully explain it a little better:

interface IFoo {
    IFoo Bar {get; set;}
}

class Parent {

}

class Child1 : Parent, IFoo {

}

class Child2 : Parent, IFoo {

}

class Manager {
    List<Parent> Parents = new List<Parent>();

    Parents.Add(new Child1());
    Parents.Add(new Child2());

    // I want to do something like this:
    Parents[0].Bar = Parents[1];
}

I believe (Parents[0] is IFoo) will evaluate as true , and yet I can't use Parents[0].Bar because Parent does not contain a definition for Bar . Can you explain why this is? Also, how can I effectively do the same thing that (at least in my head) Parents[0].Bar = Parents[1] should do?

Hopefully this makes sense. I'll try to clarify things if needed. Thanks in advance for taking a look!

Answer 1

Use type casting:

((IFoo)Parents[0]).Bar

But actually it is bad design in your case if you depend on inplementation.

Answer 2

If you want to do the assignment, you need to let your code have the same information that you have yourself, and also the same information that the Manager class has. That means that you must tell the class Parent that it knows about IFoo, and that means:

 abstract class Parent : IFoo
{
    public abstract IFoo Bar { get; set; }
}

It now also states that you can't instantiate a Parent alone. From your comments, it seems to make sense (you saying Parent should have no implementation of Bar, only know about it). If that makes sense it is abstract, but if not remove the abstract and add the implementation of Bar to Parent, or if your real implementation differs for each child, make it virtual.