// 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?)
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.
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.
You can also consider Simulated Covariance for .NET Generics by Krzysztof Cwalina
来源：https://stackoverflow.com/questions/1443341/explicit-casting-problem