Setting a property in the constructor

I actually come from Java, so that may be not really right, but I try my best.

Yes ICollection is an interface. But we cannot write

// wrong, won't work
public Series {
ComicBooks = new ICollection<ComicBook>()
}

That won't work. Because for every interface we have to provide implementation in the end.

In the way he wrote his code we want to say roughly: "Hey I know that in the end we will use List<ComicBook>, but because we define this List through ICollection, List cannot use his own methods, he can use only methods that are in ICollection"

We can say that List is implemented through interface. It is very common technique in OOP.

BUT if you decide to

Why not just use List<T> in the navigation property if we're going to instantiate it that way every time?

Then you cannot switch to new implementation by changing one line, because it can happen that some methods that are in List they are no longer in LinkedList

PS

There are ton of resources out there, try googling.

Here is one of the examples

http://stackoverflow.com/questions/383947/what-does-it-mean-to-program-to-an-interface

And one more note

It is does not matter what we use in Constructor since we will be using ONLY methods of an interface and not the concrete implementations. In the reality we instantiate ICollection in constructor so that later we don't have NullPointerException, but always have empty Collection. This is important for Database. I think.

And like I sais before later on if you decide to switch to LinkedList. It is very common.

You can just say

            ICollection<ComicBook> books = Series.ComicBooks;
            List<ComicBook> list = new List<string>(books);
            LinkedList<ComicBook> linkedList = new LinkedList<string>(books);

This should be valid code, or at least concept, i didn't check syntax.

That means EVERY time you ask for ComicBooks from Series, you get ICollection. You never get List That is why we have flexibility.

If we define List<ComicBook> then it won't work.