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

If you have a list of type List you can call list.add(new Baz()); assuming Baz implements IFoo. However you can't do that with a List, so you can't use a List everywhere you can use a List.

However since Bar implements IFoo, you can use a Bar everywhere you use IFoo, so passing a Bar to add works when it expects and IFoo.