我想使用包含的泛型進行以下繼承，但最終的轉換
a as A<XBase>總是導致 null，因為轉換無效。任何人都可以詳細說明為什么這個演員會無效，以及這個問題的解決方案。
public class XBase {}
public interface A<T> where T : XBase
{
//Edited
void Method(T param);
}
public class Implementor : A<Implementor.ImplementorX >
{
public class ImplementorX : XBase {public int a;}
//Edited
void Method(ImplementorX param) {}
}
public class HelloWorld
{
public static void Main(string[] args)
{
var a = new Implementor();
var castRes = a as A<XBase>;
Console.WriteLine(castRes != null);
}
}
查看實時示例https://rextester.com/BTNVT61833
已編輯：向 bc 添加了一個方法，interface A<T>否則它可以通過 @DavidG 的回應來解決
uj5u.com熱心網友回復：
如果您進行顯式轉換：
var castRes = A<XBase>(a);
然后你會看到以下錯誤：
無法將“”型別的物件轉換為“”型別
為什么？在我看來，最好用現實世界的例子來理解。我已經根據這個解釋重命名了類。有些評論將解釋映射到您的相關課程。
抽象：
// XBase
public class Animal { }
// class ImplementorX : XBase {public int a;}
public class Bird : Animal
{
public string WingColor { get; set; }
}
// interface A<T> where T : XBase
public interface IHat<T> where T : Animal
{
void Hide(T param);
T Pull();
}
具體實作：
// class Implementor : A<Implementor.ImplementorX >
public class Peacock : IHat<Bird>
{
// void Method(ImplementorX param) {}
void IHat<Bird>.Hide(Bird param)
{ }
public Bird Pull()
{ }
}
以及如何呼叫它：
public static void Main(string[] args)
{
Peacock peacockHat = new Peacock();
IHat<Animal> animalHat = (IHat<Animal>) peacockHat; // runtime error 'Unable to cast
// object of type 'HelloWorld.Peacock' to type 'HelloWorld.IHat`1
// because
animalHat.Hide(new Dolphin()); // Hide a dolphin in a peacock hat?
}
So we cannot hide hat of Peacock from Dolphin. It is not okay. CLR prevents us from making inappropriate actions.
In short:
In short, imagine you have two animals such as Wolf and Sheep. And these classes implements IAnimal interface:
public interface IAnimal
{ }
public class Wolf: IAnimal
{ }
public class Sheep : IAnimal
{ }
So Sheep, Wolf classes implement the inherited interface IAnimal:
IAnimal
/ \
/ \
Sheep Wolf
And then these animals can be put in cage:
public class Cage<T> where T : IAnimal
{
public void Put(T animal)
{ }
}
Then you create a cage for Sheep. After that somebody wants to cast Sheep cage to IAnimal:
Cage<Sheep> sheepCage = new Cage<Sheep>();
sheepCage.Put(new Sheep());
Cage<IAnimal> animalCage = (Cage<Wolf>)sheepCage; // compile error
// if there were no error, then you will be able to do:
animalCage.Put(new Wolf()); // it is not good idea
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