Is there any way to accomplish something like List<object> list = new List<int>() in C#?

Question

That is the question. What I want to accomplish is something similar to the following Java code:

class A { }
class B extends A { }

public class Tests {
    public static void main(String [] args) {
        ArrayList<? extends A> lists = new ArrayList<B>(); 
    }
}

(in which B extends A means B inherits from A)

Is it possible at all?

Thanks

Answer 1

No. While C# 4.0 introduces the concept of variance for interfaces, it isn't (and can't be) possible to do what you're asking specifically.

In C# 4.0, you can do this:

IEnumerable<object> foo = new List<int>();

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Edit: As Marc points out, this can't be done with value types and reference Types. However, since I think your question was more about a generalized A:B than it was about int:object, I think the point carries. To be accurate, though, what I mean is that C# 4.0 will allow something this:

IEnumerable<object> foo = new List<string>();

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But you can't do what you describe. Namely, let's consider:

List<int> foo = new List<int>();
List<object> bar = foo;

bar.Add("baz");

Now, what happens when we try to access foo[0]? You've now broken the type safety on the list.

Answer 2

Your best bet with List<object> would be to just use the non-generic IList:

IList list = new List<int>();

However! Adding something other than int will cause a runtime exception.