这是我的代码的简化版本:
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
}
}
我的线路有一个错误:
AppleBox.ChooseFirst(appleBoxes);
无法从
System.Collections.Generic.List<AppleBox>
转换为System.Collections.Generic.List<FruitBox<Apple>>
我尝试过:
AppleBox.ChooseFirst((List<FruitBox<Apple>>)appleBoxes);
但同样的错误。
我必须如何继续?
这里解释了这种行为的原因。简而言之,类不支持C#中的方差,并且List<AppleBox>
不是List<FruitBox<Apple>>
。
你能做什么:
- "转换";集合(实际创建一个新集合(:
使用OfType<>().ToList()
AppleBox.ChooseFirst(appleBoxes.OfType<FruitBox<Apple>>().ToList())
或仅ToList
AppleBox.ChooseFirst(appleBoxes.ToList<FruitBox<Apple>>())
- 更改
ChooseFirst
签名以使用协变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;
}
}
您必须将派生类的引用保存到基类变量中
List<FruitBox<Apple>> appleBoxes = new List<AppleBox>();
FruitBox<Apple> appleBox = new AppleBox();
appleBoxes.Add(appleBox);
appleBox.ChooseFirst(appleBoxes);