Why does a generic model not support covariance?

Hypothesis

Class S {} Class A: s {} Class B: s {}

The following code can be compiled

S [] array = new A [5]; array [0] = new B (); List <S> List = new list <A> ();

Running result:

The first line of code can be compiled.

An error is reported when the second line of code is run.

The third line of code fails to be compiled.

Question 1: Why does the array support covariance?

Because Java has a covariant array, the first version of. Net designers also adopts the same policy. Although this function is a common defect in Java"

Question 2: Why does a generic model not support covariance?

The generic designer thinks that it is better to fail during compilation than to run it. the full significance of the static type language lies in identifying errors before the code runs.