协方差和逆变现实世界的例子 [英] Covariance and contravariance real world example

问题描述

我有一个小麻烦理解我怎么会用协变和逆变在现实世界中。

I'm having a little trouble understanding how I would use covariance and contravariance in the real world.

到目前为止，我已经看到过相同的旧阵列例如唯一的例子。

So far, the only examples I've seen have been the same old array example.

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

这将是很好看一个例子，让我到我的开发过程中使用它，如果我能看到它在别处使用。

It would be nice to see an example that would allow me to use it during my development if I could see it being used elsewhere.

推荐答案

让我们假设你有一个类Person和提炼出来的，老师一类。你有一些操作采取的IEnumerable＆LT;＆人GT; 作为参数。在学校类，你有一个返回值 A方法IEnumerable＆LT;教师与GT; 。协方差，您可以直接使用该结果对于采取方法的IEnumerable＆LT;人＆GT; ，一个不太导出（更通用）取代更派生类型类型。逆变，与直觉相​​反，让你使用更通用的类型，其中指定了多个派生型。另请参见 https://msdn.microsoft.com/en-us/library/dd799517的.aspx

Let's say you have a class Person and a class that derives from it, Teacher. You have some operations that take an IEnumerable<Person> as the argument. In your School class you have a method that returns an IEnumerable<Teacher>. Covariance allows you to directly use that result for the methods that take an IEnumerable<Person>, substituting a more derived type for a less derived (more generic) type. Contravariance, counter-intuitively, allows you to use a more generic type, where a more derived type is specified. See also https://msdn.microsoft.com/en-us/library/dd799517.aspx

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);
    }
}

...

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());

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