列表<Type>到 IList<BaseType>

Question

I have a few classes:

class Vehicle
{
}

class Car : Vehicle
{
}

I have a list of the derived class: IList<Car> cars;

I would like to convert the list to its base class, and have tried: IList<Vehicle> baseList = cars as IList<Vehicle>;

But I always get null . Also

cars is IList<Vehicle> evaluates to be false.

Granted, I can add the items to a list if I do the following:

List<Vehicle> test = new List<Vehicle> ();

foreach ( Car car in cars )
{
   test.Add(car);
}

And I get my list, but I know there has to be a better way. Any thoughts?

Answer 1

Use IEnumerable <T> .Cast :

IList<Vehicle> vehicles = cars.Cast<Vehicle>().ToList();

Alternatively, you may be able to avoid the conversion to List depending on how you wish to process the source car list.

Answer 2

那种让您将IList<Car>为IList<Vehicle>的多态性是不安全的，因为它会让您在IList<Car>插入Truck 。

Answer 3

You're facing the problem that there is limited co- and contravariance in C#. There is an interesting approach in C# 4.0, described here at the very ending . However, it creates some other limitations that are related to the truck-problem in the answer from Novelocrat.

Answer 4

Here are a couple of approaches using Linq:

IList<Derived> list = new List<Derived>();
list.Add(new Derived());

IList<Base> otherlist = new List<Base>(from item in list select item as Base);
IList<Base> otherlist2 = new List<Base>(list.Select(item => item as Base));

Answer 5

您还可以查看 Krzysztof 的 Cwalina 文章Simulated Covariance for .NET Generics

Answer 6

var vehicles = cars.OfType<IVehicle>()

Answer 7

If you must use IList all of the way, then you are out of luck and the answers above can help you. However, if you can use an IList that is casted as IEnumerable and then simply re-casted at the destination as IList , that would work, since IEnumerable can accept such practice.

// At the source or at the model.
IEnumerable<BaseType> list = new List<Type>();
// At the destination.
IList<BaseType> castedList = (IList<BaseType>)list;

Although, since the compiler cannot enforce these things, you must manually make sure that the types and base types indeed match, of course.

Answer 8

Note that IReadOnlyList<T> from .NET 4.5+ will allow you to cast IReadOnlyList<Car> into IReadOnlyList<Vehicle> with no problems. List<T> and Collection<T> implement this interface.