Question

我将我的类层次结构定义为Object-＆gt; Fruit-＆gt; 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();
    }

现在我宣布一个代表没有任何 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，因此不应该将ApplesProcessor指定给myFunc委托。但这是可能的，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。

怎么可能？

Answer 1

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代理返回类型的协方差和委托输入类型的逆变，暗示自.NET 3.5。因此，向您的委托添加in和out没有任何区别。

添加这个的一个原因是你可以分配一个＆＃34;香草＆＃34;事件处理程序，它接受EventArgs参数到多个事件，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处理按钮单击和按键，即使前者传递MouseEventArgs参数，而后者传递KeyEventArgs参数。

逆变委托输出结果

2 个答案: