Why can't List = List?

Question

Why won\'t the following code work?

class parent {}
class kid:parent {}

List parents=new List;

It seems obvious t

Answer 1

Besides the lack of generic variance support in C# prior to version 4.0, List is mutable and so cannot safely be covariant. Consider this:

void AddOne(List arg) where T : new()
{
    arg.Add(new T());
}

void Whoops()
{
    List contradiction = new List();
    AddOne(contradiction);  // what should this do?
}

This would attempt to add a Parent to a List referenced via a List reference, which is not safe. Arrays are permitted to be covariant by the runtime, but are type-checked at mutation time and an exception is thrown if the new element is not assignable to the array's runtime element type.