协方差和逆变现实世界的例子

Question

我在理解如何在现实世界中使用协方差和逆变时遇到了一些麻烦.

到目前为止,我见过的唯一例子是同样的旧数组示例.

object[] objectArray = new string[] { "string 1", "string 2" };

很高兴看到一个允许我在开发过程中使用它的例子,如果我能看到它在其他地方使用的话.

Answer 1

// Contravariance
interface IGobbler<in T> {
    void gobble(T t);
}

// Since a QuadrupedGobbler can gobble any four-footed
// creature, it is OK to treat it as a donkey gobbler.
IGobbler<Donkey> dg = new QuadrupedGobbler();
dg.gobble(MyDonkey());

// Covariance
interface ISpewer<out T> {
    T spew();
}

// A MouseSpewer obviously spews rodents (all mice are
// rodents), so we can treat it as a rodent spewer.
ISpewer<Rodent> rs = new MouseSpewer();
Rodent r = rs.spew();

为了完整......

// Invariance
interface IHat<T> {
    void hide(T t);
    T pull();
}

// A RabbitHat…
IHat<Rabbit> rHat = RabbitHat();

// …cannot be treated covariantly as a mammal hat…
IHat<Mammal> mHat = rHat;      // Compiler error
// …because…
mHat.hide(new Dolphin());      // Hide a dolphin in a rabbit hat??

// It also cannot be treated contravariantly as a cottontail hat…
IHat<CottonTail> cHat = rHat;  // Compiler error
// …because…
rHat.hide(new MarshRabbit());
cHat.pull();                   // Pull a marsh rabbit out of a cottontail hat??

Answer 2

这是我放在一起帮助我理解差异的原因

public interface ICovariant<out T> { }
public interface IContravariant<in T> { }

public class Covariant<T> : ICovariant<T> { }
public class Contravariant<T> : IContravariant<T> { }

public class Fruit { }
public class Apple : Fruit { }

public class TheInsAndOuts
{
    public void Covariance()
    {
        ICovariant<Fruit> fruit = new Covariant<Fruit>();
        ICovariant<Apple> apple = new Covariant<Apple>();

        Covariant(fruit);
        Covariant(apple); //apple is being upcasted to fruit, without the out keyword this will not compile
    }

    public void Contravariance()
    {
        IContravariant<Fruit> fruit = new Contravariant<Fruit>();
        IContravariant<Apple> apple = new Contravariant<Apple>();

        Contravariant(fruit); //fruit is being downcasted to apple, without the in keyword this will not compile
        Contravariant(apple);
    }

    public void Covariant(ICovariant<Fruit> fruit) { }

    public void Contravariant(IContravariant<Apple> apple) { }
}

tldr

ICovariant<Fruit> apple = new Covariant<Apple>(); //because it's covariant
IContravariant<Apple> fruit = new Contravariant<Fruit>(); //because it's contravariant

Answer 3

假设你有一个类Person和一个派生自它的类,教师.你有一些IEnumerable<Person>作为参数的操作.在您的School类中,您有一个返回的方法IEnumerable<Teacher>.协方差允许您直接将该结果用于采用a的方法,将IEnumerable<Person>更多派生类型替换为较少派生(更通用)的类型.反直觉,反直觉,允许您使用更通用的类型,其中指定更多派生类型.

另请参阅MSDN上的泛型中的协方差和逆变.

课程:

public class Person 
{
     public string Name { get; set; }
} 

public class Teacher : Person { } 

public class MailingList
{
    public void Add(IEnumerable<out Person> people) { ... }
}

public class School
{
    public IEnumerable<Teacher> GetTeachers() { ... }
}

public class PersonNameComparer : IComparer<Person>
{
    public int Compare(Person a, Person b) 
    { 
        if (a == null) return b == null ? 0 : -1;
        return b == null ? 1 : Compare(a,b);
    }

    private int Compare(string a, string b)
    {
        if (a == null) return b == null ? 0 : -1;
        return b == null ? 1 : a.CompareTo(b);
    }
}

用法:

var teachers = school.GetTeachers();
var mailingList = new MailingList();

// Add() is covariant, we can use a more derived type
mailingList.Add(teachers);

// the Set<T> constructor uses a contravariant interface, IComparer<T>,
// we can use a more generic type than required.
// See https://msdn.microsoft.com/en-us/library/8ehhxeaf.aspx for declaration syntax
var teacherSet = new SortedSet<Teachers>(teachers, new PersonNameComparer());

Answer 4

in和out关键字使用泛型参数控制编译器的接口和委托的转换规则:

interface IInvariant<T> {
    // This interface can not be implicitly cast AT ALL
    // Used for non-readonly collections
    IList<T> GetList { get; }
    // Used when T is used as both argument *and* return type
    T Method(T argument);
}//interface

interface ICovariant<out T> {
    // This interface can be implicitly cast to LESS DERIVED (upcasting)
    // Used for readonly collections
    IEnumerable<T> GetList { get; }
    // Used when T is used as return type
    T Method();
}//interface

interface IContravariant<in T> {
    // This interface can be implicitly cast to MORE DERIVED (downcasting)
    // Usually means T is used as argument
    void Method(T argument);
}//interface

class Casting {

    IInvariant<Animal> invariantAnimal;
    ICovariant<Animal> covariantAnimal;
    IContravariant<Animal> contravariantAnimal;

    IInvariant<Fish> invariantFish;
    ICovariant<Fish> covariantFish;
    IContravariant<Fish> contravariantFish;

    public void Go() {

        // NOT ALLOWED invariants do *not* allow implicit casting:
        invariantAnimal = invariantFish; 
        invariantFish = invariantAnimal; // NOT ALLOWED

        // ALLOWED covariants *allow* implicit upcasting:
        covariantAnimal = covariantFish; 
        // NOT ALLOWED covariants do *not* allow implicit downcasting:
        covariantFish = covariantAnimal; 

        // NOT ALLOWED contravariants do *not* allow implicit upcasting:
        contravariantAnimal = contravariantFish; 
        // ALLOWED contravariants *allow* implicit downcasting
        contravariantFish = contravariantAnimal; 

    }//method

}//class

// .NET Framework Examples:
public interface IList<T> : ICollection<T>, IEnumerable<T>, IEnumerable { }
public interface IEnumerable<out T> : IEnumerable { }


class Delegates {

    // When T is used as both "in" (argument) and "out" (return value)
    delegate T Invariant<T>(T argument);

    // When T is used as "out" (return value) only
    delegate T Covariant<out T>();

    // When T is used as "in" (argument) only
    delegate void Contravariant<in T>(T argument);

    // Confusing
    delegate T CovariantBoth<out T>(T argument);

    // Confusing
    delegate T ContravariantBoth<in T>(T argument);

    // From .NET Framework:
    public delegate void Action<in T>(T obj);
    public delegate TResult Func<in T, out TResult>(T arg);

}//class

Answer 5

这是一个使用继承层次结构的简单示例.

给定简单的类层次结构:

public abstract class LifeForm  { }
public abstract class Animal : LifeForm { }
public class Giraffe : Animal { }
public class Zebra : Animal { }

在代码中:

public static void PrintLifeForms(IList<LifeForm> lifeForms)
{
    foreach (var lifeForm in lifeForms)
    {
        Console.WriteLine(lifeForm.GetType().ToString());
    }
}

不变性(即通用类型参数*不*用in或out关键字装饰)

貌似,这样的方法

var myAnimals = new List<LifeForm>
{
    new Giraffe(),
    new Zebra()
};
PrintLifeForms(myAnimals); // Giraffe, Zebra

...应该接受异构集合:(它确实如此)

var myGiraffes = new List<Giraffe>
{
    new Giraffe(), // "Jerry"
    new Giraffe() // "Melman"
};
PrintLifeForms(myGiraffes); // Compile Error!

但是,传递更多派生类型的集合失败!

 public static void PrintLifeForms(IList<LifeForm> lifeForms)
 {
     lifeForms.Add(new Zebra());
 }

cannot convert from 'System.Collections.Generic.List<Giraffe>' to 'System.Collections.Generic.IList<LifeForm>'

为什么？因为泛型参数IList<LifeForm>不是协变的 - IList<T>是不变的,所以IList<LifeForm>只接受参数化类型T必须的集合(实现IList)LifeForm.

如果方法实现PrintLifeForms是恶意的(但具有相同的方法签名),编译器阻止传递的原因List<Giraffe>变得明显:

public static void PrintLifeForms(IEnumerable<LifeForm> lifeForms)
{
    foreach (var lifeForm in lifeForms)
    {
        Console.WriteLine(lifeForm.GetType().ToString());
    }
}

由于IList允许添加或删除元素,LifeForm因此可以将任何子类添加到参数中lifeForms,并且违反传递给该方法的任何派生类型集合的类型.(在这里,恶意方法将尝试添加Zebra到var myGiraffes).幸运的是,编译器保护我们免受这种危险.

协方差(通用参数化类型装饰out)

协方差广泛用于不可变集合(即无法在集合中添加或删除新元素)

上述示例的解决方案是确保使用协变泛型集合类型,例如IEnumerable(定义为IEnumerable<out T>).IEnumerable没有方法可以更改为集合,并且作为out协方差的结果,任何具有子类型的集合LifeForm现在都可以传递给方法:

public void PerformZebraAction(Action<Zebra> zebraAction)
{
    var zebra = new Zebra();
    zebraAction(zebra);
}

PrintLifeForms现在可以调用Zebras,Giraffes以及IEnumerable<>任何子类LifeForm

逆变(通用参数化类型装饰in)

当函数作为参数传递时,经常使用逆变量.

这是一个函数的示例,它将Action<Zebra>一个参数作为参数,并在已知的Zebra实例上调用它:

var myAction = new Action<Zebra>(z => Console.WriteLine("I'm a zebra"));
PerformZebraAction(myAction); // I'm a zebra

正如所料,这很好用:

var myAction = new Action<Giraffe>(g => Console.WriteLine("I'm a giraffe"));
PerformZebraAction(myAction); 

直观地说,这将失败:

var myAction = new Action<Animal>(a => Console.WriteLine("I'm an animal"));
PerformZebraAction(myAction); // I'm an animal

cannot convert from 'System.Action<Giraffe>' to 'System.Action<Zebra>'

但是,这成功了

var myAction = new Action<object>(a => Console.WriteLine("I'm an amoeba"));
PerformZebraAction(myAction); // I'm an amoeba

甚至这也成功了:

public abstract class LifeForm  { }
public abstract class Animal : LifeForm { }
public class Giraffe : Animal { }
public class Zebra : Animal { }

为什么？因为Action被定义为Action<in T>,即它contravariant,意味着,对于Action<Zebra> myAction,myAction可以是"最多"的Action<Zebra>,但是较少派生的超类Zebra也是可接受的.

虽然这一开始可能不直观(例如,如何将Action<object>其作为参数传递Action<Zebra>？),如果解压缩步骤,您会注意到被调用的函数(PerformZebraAction)本身负责传递数据(在本例中是一个Zebra实例) )函数 - 数据不是来自调用代码.

由于以这种方式使用高阶函数的反向方法,在Action调用时,它是Zebra针对zebraAction函数调用的更多派生实例(作为参数传递),尽管函数本身使用较少派生的类型.

Answer 6

class A {}
class B : A {}

public void SomeFunction()
{
    var someListOfB = new List<B>();
    someListOfB.Add(new B());
    someListOfB.Add(new B());
    someListOfB.Add(new B());
    SomeFunctionThatTakesA(someListOfB);
}

public void SomeFunctionThatTakesA(IEnumerable<A> input)
{
    // Before C# 4, you couldn't pass in List<B>:
    // cannot convert from
    // 'System.Collections.Generic.List<ConsoleApplication1.B>' to
    // 'System.Collections.Generic.IEnumerable<ConsoleApplication1.A>'
}

基本上每当你有一个带有一个类型的Enumerable的函数时,如果没有显式地转换它,你就无法传入一个派生类型的Enumerable.

只是为了警告你一个陷阱:

var ListOfB = new List<B>();
if(ListOfB is IEnumerable<A>)
{
    // In C# 4, this branch will
    // execute...
    Console.Write("It is A");
}
else if (ListOfB is IEnumerable<B>)
{
    // ...but in C# 3 and earlier,
    // this one will execute instead.
    Console.Write("It is B");
}

无论如何这都是可怕的代码,但它确实存在,如果使用这样的结构,C#4中不断变化的行为可能会引入微妙且难以发现的错误.

Answer 7

逆变

在现实世界中，您始终可以使用动物收容所而不是兔子收容所，因为每次动物收容所收容一只兔子时，它都是动物。但是，如果您使用兔子收容所而不是动物收容所，它的工作人员可能会被老虎吃掉。

在代码中，这意味着如果你有一个，IShelter<Animal> animals你可以简单地编写IShelter<Rabbit> rabbits = animals if你 promise 和 use Tin IShelter<T>only 作为方法参数，如下所示：

public class Contravariance
{
    public class Animal { }
    public class Rabbit : Animal { }

    public interface IShelter<in T>
    {
        void Host(T thing);
    }

    public void NoCompileErrors()
    {
        IShelter<Animal> animals = null;
        IShelter<Rabbit> rabbits = null;

        rabbits = animals;
    }
}

并用更通用的项目替换项目，即减少方差或引入反向方差。

协方差

在现实世界中，您始终可以使用兔子供应商而不是动物供应商，因为每次兔子供应商给您一只兔子时，它都是动物。但是，如果您使用动物供应商而不是兔子供应商，您可能会被老虎吃掉。

在代码中，这意味着如果你有一个，ISupply<Rabbit> rabbits你可以简单地写ISupply<Animal> animals = rabbits if you promise 和 use Tin ISupply<T>only as 方法返回值，如下所示：

public class Covariance
{
    public class Animal { }
    public class Rabbit : Animal { }

    public interface ISupply<out T>
    {
        T Get();
    }

    public void NoCompileErrors()
    {
        ISupply<Animal> animals = null;
        ISupply<Rabbit> rabbits = null;

        animals = rabbits;
    }
}

并用更派生的项目替换项目，即增加方差或引入协方差。

总而言之，这只是您的一个编译时可检查承诺，即您将以某种方式对待泛型类型以保持类型安全并且不会让任何人吃掉。

你可能想读一读这篇文章，让你的头脑围绕这一点。

Answer 8

来自MSDN

以下代码示例显示了对方法组的协方差和逆变支持

static object GetObject() { return null; }
static void SetObject(object obj) { }

static string GetString() { return ""; }
static void SetString(string str) { }

static void Test()
{
    // Covariance. A delegate specifies a return type as object, 
    // but you can assign a method that returns a string.
    Func<object> del = GetString;

    // Contravariance. A delegate specifies a parameter type as string, 
    // but you can assign a method that takes an object.
    Action<string> del2 = SetObject;
}