Question

There are some great resources on covariance and contravariance here on StackOverflow, but I seem to misunderstand the fundamentals of contravariance. I expect this example to work:

    public partial class WebForm1 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        A a = new B();
        B b = new A();
    }
}

public class A
{
    int id { get; set; }
}
public class B : A
{
}

Setting a to B works, which is covariance, but setting b to a new A fails with a compile error. Even doing an explicit cast still generates error at compile time. Is there a way to do this or do I just completely misunderstand contravariance?

Answer 1

You can’t do:

B b = new A();

Because an A simply isn’t a B. The assignment is invalid. I’m not sure I’d even call this variance – it is simply inheritance.

In the general case where B had members that A doesn’t, you can see that it makes no sense to do (if b actually holds a reference to an A object):

b.SomeMethod();

where SomeMethod is defined only in B, but this logic extends to the assignment to the variable itself. Even if you added a cast, the cast has an implicit type check that would fail.