Question

I’m just wondering why passing a System.Collections.Generic.List<string> into this function test(ICollection<object> t) will not work, why can’t I pass it in like passing a string into test2(object t)?

Doesn’t make much sense to me!

Answer 1

Because ICollection isn’t an output only interface, it is not covariant.

Consider this code:

void test(ICollection<object> t)
{
    t.Add(new TextBox());
}

List<string> lst;
test(lst);

What is supposed to happen when test tries to stuff a TextBox into a List<string>?

The contract for ICollection<object> is that any object can be put in, and items coming out will always be of type object. But List<string> only meets half that contract.