C# List: why you cannot do `List foo = new List();`

Question

If you have an Interface IFoo and a class Bar : IFoo, why can you do the following:

List foo = new List();          


        

Answer 1

Because a list of IFoos can contain some Bars as well, but a list of IFoos is not the same thing as a list of Bars.

Note that I used English above instead of using C#. I want to highlight that this is not a deep problem; you are just getting confused by the details of the syntax. To understand the answer you need to see beyond the syntax and think about what it actually means.

A list of IFoos can contain a Bar, because a Bar is an IFoo as well. Here we're talking about the elements of the list. The list is still a list of IFoos. We haven't changed that.

Now, the list you called foo is still a list of IFoos (more pedantically, foo is declared as a List). It cannot be anything else. In particular, it cannot be made into a list of Bars (List). A list of Bar is a completely different object than a list of IFoos.