Storing list of generic class of derived objects

问题

For example, how do I store a list of DataContainers that all use types that derive from the same base class.

public class Animal {}
public class Cat : Animal {}
public class Dog : Animal {}
public class DataContainer <TData> where TData : Animal {
    TData innerObject = new TData ();
    public TData GetData () {
        return innerObject;
    }
}

public class DataManager {
    static void Main () {
        DataContainer<Cat> CatData = new DataContainer<Cat> ();
        DataContainer<Dog> DogData = new DataContainer<Dog> ();
        var AnimalData = new List<DataContainer<Animal>> ();
        AnimalData.Add (CatData);
        AnimalData.Add (DogData);
        for (int i = 0; i < AnimalData.Count; i++) {
            Animal animal = AnimalData[i].GetData ();
        }
    }
}

If it's not possible with a generic class, could it be possible with Arrays?

Edit: Getting this error: ArrayTypeMismatchException: Source array type cannot be assigned to destination array type. This causes an ArrayTypeMismatchException:

public interface IDataContainer <out TData> where TData : Animal {

}
public class DataContainer <TData> : IDataContainer<TData> where TData : Animal {

}

public class Tester () {
    static void Main () {
        var AnimalData = new List<DataContainer<Animal>> ();
        var CatData = new DataContainer<Cat> ();
        AnimalData.Add (CatData as IDataContainer<Animal>); //Error
    }
}

Run in Unity 5.2

回答1:

The issue is that you need your generic class to have a covariant type parameter.

You can only do this on interfaces, so create one:

public interface IDataContainer <out TData> where TData : Animal 
{
    TData GetData ();
}

Note the use of out to mark TData as covariant. Now have your DataContainer class implement the interface, and when you hold onto a reference in the client code, make sure its an IDataContainer. Now it should let you store it as you expected.

Do note that covariant interfaces have special requirements, namely that the generic type parameter can not be used as the argument type for any of the methods (see the MSDN link for more).

来源：https://stackoverflow.com/questions/33181041/storing-list-of-generic-class-of-derived-objects

标签

generics

inheritance

polymorphism