逆变委托输出结果

Question

我将 class 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 classes

    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，我做了以下分配：

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

因为我没有将 TResult 声明为协变 out 参数，所以应该不可能将 ApplesProcessor 分配给 myFunc 委托。但有可能，程序编译和执行时没有任何错误。

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。

怎么可能？

Answer 1

像这样的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，您正在将非通用委托分配给通用委托 - 在这种情况下始终使用隐式转换。要真正让你的代码失败，你需要做这样的事情：

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

Answer 2

根据 MSDN，自 .NET 3.5 以来，委托 return 类型的协变和委托输入类型的逆变是隐含的。因此，将 in 和 out 添加到您的委托中没有任何区别。

添加此选项的一个原因是您可以将接受 EventArgs 参数的 "vanilla" 事件处理程序分配给多个事件，这些事件可能使用更具体的类派生自 EventArgs。例如，您可以使用同一个委托来处理按钮单击和按键按下，即使前者传递了 MouseEventArgs 参数而后者传递了 KeyEventArgs 参数。

逆变委托输出结果

Contravariant delegate output result

c#

delegates

covariance