如何将子类的字典视为基类的字典 [英] How to treat a dictionary of subclasses as a dictionary of the base class
问题描述
在C#中,我有一堆对象都继承自同一基类.
我也有许多词典,每个子类一个.
我想做的就是将所有这些词典添加到一个 List 中,这样我就可以遍历它们并做一些工作(例如比较列表等).
In C# I have a bunch of objects all inheriting from the same base class.
I also have a number of Dictionaries, one for each subclass.
What I want to do is add all of those Dictionaries to one List so I can loop through them all and do some work (like comparing the lists etc).
总结
Dictionary<string, Child> childObjects = new Dictionary<string, Child>();
List<Dictionary<string, Parent>> listOfDictionaries = new List<Dictionary<string, Parent>>();
listOfDictionaries.Add(childObjects);
我本以为,由于Child从Parent继承而来,所以应该可以,但是不会编译.显然,我对继承和泛型不了解:)
I would have thought that since Child inherits from Parent, this should work, but it won't compile. Clearly I am not understanding something about inheritance and generics :)
完整的代码示例
class Program
{
static void Main(string[] args)
{
//Creating a Dictionary with a child object in it
Dictionary<string, Child> childObjects = new Dictionary<string, Child>();
var child = new Child();
childObjects.Add(child.id, child);
//Creating a "parent" Dictionary with a parent and a child object in it
Dictionary<string, Parent> parentObjects = new Dictionary<string, Parent>();
parentObjects.Add(child.id, child);
var parent = new Parent();
parentObjects.Add(parent.id, parent);
//Adding both dictionaries to a general list
List<Dictionary<string, Parent>> listOfDictionaries = new List<Dictionary<string, Parent>>();
listOfDictionaries.Add(childObjects); //This line won't compile
listOfDictionaries.Add(parentObjects);
}
}
class Parent
{
public string id { get; set; }
public Parent()
{
this.id = "1";
}
}
class Child : Parent
{
public Child()
{
this.id = "2";
}
}
有什么办法可以做到这一点?
Is there any way of achieving this?
推荐答案
您不能安全地执行此操作.假设您这样做:
You can't do this safely. Imagine you did this:
listOfDictionaries[0]["foo"] = new Parent();
看起来 很好-但这意味着childObjects
将包含一个不是Child
实例的值!
That looks fine - but it would mean that childObjects
would contain a value which isn't an instance of Child
!
C#4在安全处引入了受限的泛型方差-例如,您可以将类型IEnumerable<Banana>
的引用转换为IEnumerable<Fruit>
-但是您要在此处执行的操作是'是安全的,因此仍然不允许.
C# 4 has introduced restricted generic variance where it's safe - so you can convert a reference of type IEnumerable<Banana>
to IEnumerable<Fruit>
for example - but what you're wanting to do here isn't safe, so it still isn't allowed.
如果您可以向我们进一步介绍更广泛的背景信息-您想要实现的目标-我们可能会提供更多帮助.之后,您能举例说明您要如何处理列表吗?
If you could tell us more about the bigger context - what you're trying to achieve - we may be able to help more. Can you give examples of what you'd want to do with the list afterwards?
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