Passing Collection of Inherited Objects to Method Accepting an ICollection of Base Objects

Question

Background: I have a generic collection of type DirectoryCollection<T> and have specified that T must be of type IEntity. I have a concrete type Entity which implements IEntity and two derived types, Employee and Station.

In .NET 4.0 I know that it is possible (from here) to call a method with an IEnumerable<Entity> parameter as DirectoryCollection<Employee>, however, it seems that does not work for a method with a parameter of type ICollection<Entity>. In said method, I use the Remove and Add methods of the collection and consequently, am hesitant to put IEnumerable into the method signature.

What is the best practice in solving this problem?

Answer 1

You need to implement ICollection in the DirectoryCollection class. You may need to implement it explicitly, due to the covariance restriction that @SamHolder mentions in the comments. See this tutorial on implementaing an interface explicitly.

Furthermore, in the methods that accept ICollection<Entity> as a parameter, you may need to change the signature to accept ICollection<IEntity> instead.