c# - Polymorphism - What am I not getting? -

i having issue polymorphism in c#. have object implements interface, cannot represent collection of objects collection of interfaces. flies in face of understanding of polymorphism. wondering have gone wrong.

[testfixture] class tester {     [test]     public void polymorphism()     {         var list = new list<foo> {new foo {name = "item"}};          assert.that(list, is.instanceof<ilist>());         assert.that(list[0], is.instanceof<foo>());         assert.that(list[0], is.instanceof<ibar>());          // why rest true false?         assert.that(list, is.instanceof<ilist<ibar>>());     } }  internal interface ibar { }  internal class foo : ibar {     public string name { get; set; } } 

this question of variance, not polymorphism.

if list-of-foo ilist-of-ibar, following work:

class : ibar {} ilist<ibar> list = new list<foo>(); list.add(new another()); 

then we've added list of foo. error. compiler stopped making mistake.

note recent compilers / .net versions support variance via "in"/"out". list-of-foo fine ienumerable-of-ibar. because guaranteed return foo (not accept them), , foo ibar - hence safe.