Explicit Casting Problem

Question

// The Structure of the Container and the items
public interface IContainer <TItem> where TItem : IItem
{

}

public class AContainer : IContainer<ItemA>
{

}

public interface IItem
{

}

public class ItemA : IItem
{

}

// Client app

[Test]
public void Test ()
{
 IContainer<IItem> container = new AContainer();
}

Question : In test the following error occures. What can be the solution for casting?

Cannot implicitly convert type 'AContainer' to 'IContainer'. An explicit conversion exists (are you missing a cast?)

This issue seems to come up most days. Did you search before posting? — thecoop, Sep 18 '09 at 09:10

score 3 · Accepted Answer · answered Sep 18 '09 at 09:02

3

Another generics covariant problem...

Generic types in .NET are not covariant or contravariant - IContainer<ItemA> (which is what AContainer is) is not a subclass of IContainer<IItem> - there is no valid cast between the two. This will be fixed in C# 4.

answered Sep 18 '09 at 09:02

thecoop

45,220
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3

I note that in C# 4 we are adding covariance and contravariance **but** only on interfaces and delegates with reference type arguments in the varying positions, and only those interfaces and delegates which are known at compile time to be safe for variance. – Eric Lippert Sep 18 '09 at 14:59

score 1 · Answer 2 · answered Sep 18 '09 at 09:10

1

If you want to use AContainer to be an IContainer<IItem>, you need to implement this interface as well:

public class AContainer : IContainer<ItemA>, IContainer<IItem>

You may implement it explicitly.

answered Sep 18 '09 at 09:10

Stefan Steinegger

63,782
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score 0 · Answer 3 · answered Sep 18 '09 at 09:10

0

You can also consider Simulated Covariance for .NET Generics by Krzysztof Cwalina

answered Sep 18 '09 at 09:10

Dzmitry Huba

4,493
20
19

Explicit Casting Problem

3 Answers3

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