委托不接受子类？ [英] Delegate doesn't accept subclass?

问题描述

我的代表似乎不接受子类，我认为这是最简单的例子。

My delegate doens't seem to accept a subclass, I think an example is the easiest.

public class A
{
     public A() { }
}

public class B : A
{
     public B() { }
}

public class Program
{
     private delegate void CallBack(A a);
     private static CallBack callBack = new CallBack(Test);

     public Main(string[] args)
     {
          callBack(new B());
     }

     private static void Test(A a)
     {
          Console.WriteLine("Test()");    
     }

     // Compilation error occurs if Test becomes:
     private static void Test(B a)
     {
          Console.WriteLine("Test()");
     }
 }

当我更改测试以接受 B 会引发编译错误。是不是很奇怪，因为 B 扩展了 A ？

When I change Test to accept B it throws a compilation error. Isn't this odd because B extends A?

编译器错误：

测试匹配无重载

No overload for Test matches Callback

有没有办法让我的代表接受扩展 A 的类？

Is there a way to make my delegate accept a class that extends A?

推荐答案

这并不奇怪，因为如果您有扩展了的 C 类的对象A ，如果只接受 B Test（）没有任何意义$ c>。用于回调的任何方法都必须接受 any A ，而不仅仅是特定的子类。如果要 Test（，您需要更改回调委托签名以接受 B 。 ）也接受 B 。

It isn't odd because if you have an object of class C that extends A, it wouldn't make sense to pass to Test() if it only accepts a B. Any method used for a Callback has to accept any A, not just a specific subclass. You would need to change the Callback delegate signature to accept B if you want to Test() to accept B as well.

class C : A {};

Callback callback = Test;

callback(new C()); //what if Test() accepted B???

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