为什么我无法从列表< MyClass>投射列出< object&gt ;? [英] Why can't I cast from a List<MyClass> to List<object>?

问题描述

我有一个对象列表，这些对象的类型是 QuoteHeader ，我想把这个列表作为一个对象列表传递给一个能够接受列表< object> 。

我的代码行显示...

  Tools.MyMethod（（List< object>）MyListOfQuoteHeaders）; 
  

但在设计时出现以下错误...

 无法将类型'System.Collections.Generic.List< MyNameSpace.QuoteHeader>'
转换为'System.Collections.Generic.List< object>'
  

我需要为我的课程做些什么才能做到这一点？我认为所有的类都从对象继承，所以我不明白为什么这不起作用？

解决方案

原因是不合法是因为它不安全。假设它是合法的：

  List< Giraffe>长颈鹿=新列表<长颈鹿>（）; 
列表< Animal>动物=长颈鹿; //这是不合法的;假设它是。 
 animals.Add（new Tiger（））; //将老虎放入动物列表总是合法的
  

但是动物实际上是长颈鹿名单;您不能将老虎列入长颈鹿列表中。

不幸的是，在C＃中这与引用类型的数组合法：

 长颈鹿[]长颈鹿=新长颈鹿[10]; 
 Animal []动物=长颈鹿; //合法！但是很危险，因为... 
 animals [0] = new Tiger（）; // ...在运行时失败！ 
  

在C＃4中，这对于 IEnumerable 是合法的，但不是IList ：

 列表<长颈鹿>长颈鹿=新列表<长颈鹿>（）; 
 IEnumerable< Animal>动物=长颈鹿; //在C＃中的合法性4 
 foreach（Animal animals in animals）{} //每个长颈鹿都是动物，所以这是安全的
  

这是安全的，因为 IEnumerable< T> 不公开任何 / p>

要解决您的问题，您可以：

• 创建一个新的对象列表。
  
• 使方法获取一个对象[]而不是 List< object> ，并使用不安全的数组协方差。
  
• 使方法为通用方法，因此它需要一个 List< T>


• Make该方法采用IEnumerable


• 使方法采用 IEnumerable< object> 并使用C＃4。 / ul>
  

  I have a List of objects, which are of my type QuoteHeader and I want to pass this list as a list of objects to a method which is able to accept a List<object>.

  My line of code reads...

  Tools.MyMethod((List<object>)MyListOfQuoteHeaders);
  

  But I get the following error at design time...

  Cannot convert type 'System.Collections.Generic.List<MyNameSpace.QuoteHeader>' 
  to 'System.Collections.Generic.List<object>'
  

  Do I need to do anything to my class to allow this? I thought that all classes inherit from object so I can't understand why this wouldn't work?

  解决方案

  The reason this is not legal is because it is not safe. Suppose it were legal:

  List<Giraffe> giraffes = new List<Giraffe>();
  List<Animal> animals = giraffes; // this is not legal; suppose it were.
  animals.Add(new Tiger());  // it is always legal to put a tiger in a list of animals
  

  But "animals" is actually a list of giraffes; you can't put a tiger in a list of giraffes.

  In C# this is, unfortunately, legal with arrays of reference type:

  Giraffe[] giraffes = new Giraffe[10];
  Animal[] animals = giraffes; // legal! But dangerous because...
  animals[0] = new Tiger(); // ...this fails at runtime!
  

  In C# 4 this is legal on IEnumerable but not IList:

  List<Giraffe> giraffes = new List<Giraffe>();
  IEnumerable<Animal> animals = giraffes; // Legal in C# 4
  foreach(Animal animal in animals) { } // Every giraffe is an animal, so this is safe
  

  It is safe because IEnumerable<T> does not expose any method that takes in a T.

  To solve your problem you can:

  • Create a new list of objects out of the old list.
  • Make the method take an object[] rather than a List<object>, and use unsafe array covariance.
  • Make the method generic, so it takes a List<T>
  • Make the method take IEnumerable
  • Make the method take IEnumerable<object> and use C# 4.

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