C＃ - 为什么我得到一个空的收藏？ [英] C# - why am I getting an empty collection?

问题描述

为什么人收藏中有项目只有当我在子类中重写呢？下面的代码。如果我取消了重载方法，那么我的收藏有2人在公园里

 公共类公园：事情$​​ B $ B {
} 
 
公共抽象类的事情
 {
公共虚拟目录<&东西GT;人们{搞定; } =新的List<&东西GT;（）; 
} 
 
公共类的人物：事情
 {
事情磷; 
公共字符串名称{;组; } 
公众形象（事P）
 {
 this.p = P; 
 NAME =结婚; 
 this.p.people.Add（本）; 
} 
 //公众覆盖列表<&东西GT;人= GT; p.people; 
} 
 
公共类PersonB：事情
 {
事情磷; 
公共字符串名称{;组; } 
公共PersonB（事P）
 {
 this.p = P; 
 NAME =乔; 
 this.p.people.Add（本）; 
} 
 //公众覆盖列表<&东西GT;人= GT; p.people; 
} 
  

和这里的测试应用程序：

 公园东西=新园（）; 
 =园区新PERSONA（公园）; 
 =园区新PersonB（公园）; 
 
 Console.WriteLine（park.people.Count）; 
  

解决方案

 的事情公园=新园（）; 
  

下面你实例化一个公园对象，其分配给其类型的变量是的事情。到目前为止，这么好

 公园=新的人物（公园）; 
  

下面你实例化一个人物，和因为你通过公园对象的构造，该构造函数将自身添加到公园的个人集合。所以，该集合现在包含一个人。再次，到目前为止，一切都很好。

不过，你那么新的人物对象分配给公园变量。这不是一个运行时错误，因为变量的类型的事情和人物是事，但它几乎肯定你一个逻辑错误，因为我无法想象的原因，你会想要一个叫做变量公园这是指一个人。

关键一点是，在这一点上，<​​code> park.People 并非指对人的公园对象的集合;它指的是人的人物对象的集合，这是空的。

 公园=新PersonB（公园）; 
  

现在，当你调用 PersonB 构造，你不能传递一个公园对象;你传递它，你分配到公园变量人物对象。因此， PersonB 构造函数添加自己到人物的人集合，它现在包含一个人。

但同样，你分配结果公园。所以现在公园包含 PersonB 对象，其人收集空。这就是为什么这样的：

  Console.WriteLine（park.people.Count）; 
  

打印为零。

How come people collection has items in it only if I override it within the subclasses? Here's the code. If I uncomment the overridden methods, then my collection has 2 people in the park.

public class Park : Thing
{
}

public abstract class Thing
{
    public virtual List<Thing> people { get; } = new List<Thing>();
}

public class PersonA : Thing
{
    Thing p;
    public string Name { get; set; }
    public PersonA(Thing p)
    {
        this.p = p;
        Name = "Marry";
        this.p.people.Add(this);
    }
    //public override List<Thing> people => p.people;
}

public class PersonB : Thing
{
    Thing p;
    public string Name { get; set; }
    public PersonB(Thing p)
    {
        this.p = p;
        Name = "Joe";
        this.p.people.Add(this);
    }
    //public override List<Thing> people => p.people;
}

And here's the test application:

Thing park = new Park();
park = new PersonA(park);
park = new PersonB(park);

Console.WriteLine(park.people.Count);

解决方案

Thing park = new Park();

Here you are instantiating a Park object and assigning it to a variable whose type is Thing. So far, so good.

park = new PersonA(park);

Here you are instantiating a PersonA, and because you pass your Park object to the constructor, the constructor adds itself to the Park's People collection. So that collection now contains one person. Again, so far, so good.

However, you then assign the new PersonA object to the park variable. This is not a runtime error because the variable has type Thing and a PersonA is a Thing, but it's almost certainly a logic error on your part, because I can't conceive of a reason why you'd want a variable called park that refers to a person.

The critical thing is that at this point, park.People doesn't refer to the Park object's collection of people; it refers to the PersonA object's collection of people, which is empty.

park = new PersonB(park);

Now when you invoke the PersonB constructor, you're not passing it a Park object; you're passing it the PersonA object that you assigned to the park variable. So the PersonB constructor adds itself to the PersonA's People collection, which now contains one person.

But again, you're assigning the result to park. So now park contains a PersonB object whose People collection is empty. Which is why this:

Console.WriteLine(park.people.Count);

Prints zero.

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