c# Upcast 与泛型

Question

我定义了一个带有抽象约束的抽象 class :

abstract class Asset<P> where P : Parm { }
abstract class Parm { }

class StockParm : Parm { }
class Stock : Asset<StockParm> { }

class BondParm : Parm { }
class Bond : Asset<BondParm> { }

List<Asset<Parm>> assets = new List<Asset<Parm>>();
Stock stock;
assets.Add(stock);

但是我收到了错误信息：

"Cannot convert type Stock to Asset"

有没有办法列出所有 assets？

谢谢

Answer 1

你需要向编译器解释Asset<Derived>是Asset<Base>。编译器将通用类编译成 deperate 类 - 每个 T 一个。告诉编译器它是"ok"的名称是covariant。

MSDN：

Covariance - Enables you to use a more derived type than originally specified. You can assign an instance of IEnumerable (IEnumerable(Of Derived) in Visual Basic) to a variable of type IEnumerable.

将Asset<T>更改为一个接口并添加out keyword:

 interface Asset<out P> where P : Parm { }

public static class Program
{
    public static void Main(string[] args)
    {
        List<Asset<Parm>> assets = new List<Asset<Parm>>();
        Stock stock = new Stock();
        assets.Add(stock);
    }
}

public abstract class Parm { }
public class StockParm : Parm { }

public interface Asset<out P> where P : Parm { }    
public class Stock : Asset<StockParm> { }

Answer 2

请尝试以下操作

List<Asset<StockParm>> assets = new List<Asset<StockParm>>();

c# Upcast 与泛型

c# Upcast with generics

c#

generics

casting

abstract