[英]C#, How to pass a List of a Derived class to a method that receives a List of the Base class?
這是我的代碼的簡化版本:
using System.Collections.Generic;
public abstract class FruitBox<T>
{
public T item;
public static T ChooseFirst(List<FruitBox<T>> fruitBoxes)
{
return fruitBoxes[0].item;
}
}
public class Apple
{
}
public class AppleBox : FruitBox<Apple>
{
}
public class FruitShop
{
List<AppleBox> appleBoxes = new List<AppleBox>();
public void Main()
{
AppleBox appleBox = new AppleBox();
appleBoxes.Add(appleBox);
AppleBox.ChooseFirst(appleBoxes); // => Error here
}
}
我在該行中有一個錯誤:
AppleBox.ChooseFirst(appleBoxes);
無法從
System.Collections.Generic.List<AppleBox>
轉換為System.Collections.Generic.List<FruitBox<Apple>>
我試過了:
AppleBox.ChooseFirst((List<FruitBox<Apple>>)appleBoxes);
但是同樣的錯誤。
我該如何進行?
這種行為的原因在這里解釋。 簡而言之 - 類不支持C#中的差異並且List<AppleBox>
不是List<FruitBox<Apple>>
。
你可以做什么:
使用OfType<>().ToList()
AppleBox.ChooseFirst(appleBoxes.OfType<FruitBox<Apple>>().ToList())
或者只是ToList
AppleBox.ChooseFirst(appleBoxes.ToList<FruitBox<Apple>>())
ChooseFirst
簽名以使用協變IEnumerable<out T>
接口:public abstract class FruitBox<T>
{
public T item;
public static T ChooseFirst(IEnumerable<FruitBox<T>> fruitBoxes)
{
return fruitBoxes.First().item;
}
}
您必須將派生 class 的引用保存到基礎 class 變量中
List<FruitBox<Apple>> appleBoxes = new List<AppleBox>();
FruitBox<Apple> appleBox = new AppleBox();
appleBoxes.Add(appleBox);
appleBox.ChooseFirst(appleBoxes);
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