逆变委托输出结果

Question

我将我的类层次结构定义为 Object->Fruit->Apple

class Fruit : object{}

class Apple : Fruit{}

并创建了两个静态方法来处理这些类

    static Fruit FruitProcessor(string fruit)
    {
        return new Fruit();
    }

    static Apple ApplesProcessor(string apple)
    {
        return new Apple();
    }

现在我声明一个委托没有任何 in, out关键字

public delegate TResult Funk<T, TResult>(T arg);

在我的代码中，我执行以下任务：

        Funk<string, Fruit> myFunc;
        myFunc = FruitProcessor; // ok, we match signature exactly
        myFunc = ApplesProcessor;// this should not work, but works

由于我没有将 TResult 声明为协变 out 参数，因此不能将 ApplesProcessor 分配给 myFunc 委托。但有可能，程序编译和执行没有任何错误。

如果我更改 Funk 签名以添加 out TResult

public delegate TResult Funk<T, out TResult>(T arg);

一切都和以前一样。

这怎么可能？

Answer 1

这种情况由通常的隐式转换处理。引用相关的 MSDN 页面 (https://msdn.microsoft.com/en-us/library/dd233060.aspx)：

public class First { }
public class Second : First { }
public delegate First SampleDelegate(Second a);
public delegate R SampleGenericDelegate<A, R>(A a);

// Matching signature. 
public static First ASecondRFirst(Second first)
{ return new First(); }

// The return type is more derived. 
public static Second ASecondRSecond(Second second)
{ return new Second(); }

// The argument type is less derived. 
public static First AFirstRFirst(First first)
{ return new First(); }

// The return type is more derived  
// and the argument type is less derived. 
public static Second AFirstRSecond(First first)
{ return new Second(); }

// Assigning a method with a matching signature  
// to a non-generic delegate. No conversion is necessary.
SampleDelegate dNonGeneric = ASecondRFirst;
// Assigning a method with a more derived return type  
// and less derived argument type to a non-generic delegate. 
// The implicit conversion is used.
SampleDelegate dNonGenericConversion = AFirstRSecond;

// Assigning a method with a matching signature to a generic delegate. 
// No conversion is necessary.
SampleGenericDelegate<Second, First> dGeneric = ASecondRFirst;
// Assigning a method with a more derived return type  
// and less derived argument type to a generic delegate. 
// The implicit conversion is used.
SampleGenericDelegate<Second, First> dGenericConversion = AFirstRSecond;

简而言之，在您的确切情况下，您将非泛型委托分配给泛型委托 - 在这种情况下始终使用隐式转换。要真正使您的代码失败，您需要执行以下操作：

Funk<string, Fruit> myFunc;
Funk<string, Apple> myAppleFunc = ApplesProcessor;
myFunc = FruitProcessor;
myFunc = myAppleFunc; // Undefined implicit conversion on generic delegate

Answer 2

根据MSDN，从 .NET 3.5 开始隐含委托返回类型的协变和委托输入类型的逆变。因此，将 in 和 out 添加到您的委托没有任何区别。

添加它的一个原因是，您可以将接受EventArgs 参数的“普通”事件处理程序分配给多个事件，这些事件可能使用派生自EventArgs 的更具体的类。例如，您可以使用相同的委托来处理按钮单击和按键操作，即使前者传递了 MouseEventArgs 参数，而后者传递了 KeyEventArgs 参数。