Question

So I have an object, lets say Item<U>. I want to create a(n) (I)List<T> containing a collection of these items. The problem that I am running into is that the collection of Item<U> can have any number of different types for U in them.

What I can not figure out is the signature to use for the IList. I tryed IList<Item<T>>, but because I do not (in the class where the IList is to be used) have a defination of T (which as I said varys anyhow) I can not use this signature. What is the best way to approach this requirement?

Answer 1

You need to add a non-generic interface to your Item<U> and make the collection out of those interfaces.

There is no way you can create a collection of IList<Item<T>> where T is some base type for a lot of different object types you want to place into the same list. This is co(ntra)-variance in place which specifically prohibits this.

So you need to instead add something common, that isn’t generic, and make the list out of that. For instance, you might make Item<T> implement IItem, and create IList<IItem> instead.

That’s the best you can do.