ICollection的< T>不是协变？ [英] ICollection<T> not Covariant?

问题描述

这样做的目的是要同步两个集合，发送器侧和放大器;接收端，包含图形边缘，这样有事时（删除边，边加等），双方都通知。

The purpose of this is to synchronize two collections, sender-side & receiver-side, containing a graph edge, so that when something happens (remove edge, add edge, etc) both sides are notified.

要做到这一点，（背）的集合引用被包含在集合中的元素

To do so, (back-)references to the collections were included in the element in collections

class EdgeBase {
    EdgeBase(ICollection<EdgeBase> rCol, ICollection<EdgeBase> sCol)     
    { RecvCol=rCol;  SendCol=sCol; }      
    ICollection<EdgeBase> RecvCol;      
    ICollection<EdgeBase> SendCol;       
    public virtual void Disconnect() // Synchronized deletion         
    { RecvCol.Remove(this);  SendCol.Remove(this); }                 
}         
class Edge : EdgeBase {       
    Edge(ICollection<EdgeBase> rCol, ICollection<EdgeBase> sCol)     
    : base(rCol, sCol) {}
    int Weight;     
}      

删除（断开）是确定的，但创作过程中出现的问题：

Deletion (Disconnect) was ok , but the problem occurred during creation:

HashSet<Edge> receiverSet, senderSet;
var edge = new Edge(receiverSet, senderSet); // Can't convert Edge to EdgeBase!

虽然边缘是<$ C派生$ C> EdgeBase ，这是非法的。
（问题是边缘一部分，而不是的HashSet<> 部分）

Although Edge is derived from EdgeBase, this is illegal. (The problem is Edge part, not HashSet<> part.)

写数百行后，我发现了的ICollection<> 不是协变的是的IEnumerable<>

After writing hundreds of lines I found out ICollection<> is not covariant as is IEnumerable<>.

可能是什么解决办法

编辑：

如果我写了上面，而不是打破的C＃的协方差规则，它会一直像这样的代码：

If I wrote the code above while not breaking the C#'s covariance rules it would have been like this:

public class EdgeBase<T, U>
    where T : ICollection<U<T>> // illegal
    where U : EdgeBase<T, U>    // legal, but introduces self-reference
{
    public EdgeBase(T recvCol, T sendCol) {...}
    protected T ReceiverCollection;
    protected T SenderCollection;
    public virtual void Disconnect() {...}
}

不过，这是非法的; U不能与正式参数T使用。

But this is illegal; 'U' can't be used with formal parameter T.

推荐答案

的埃里克利珀说，C＃只支持类型安全的协变和逆变。如果你会想到它，使的ICollection 协不是类型安全的。

Eric Lippert said that C# will only support type-safe covariance and contravariance. If you would think of it, making ICollection covariant is not type-safe.

让我们说你有

ICollection<Dog> dogList = new List<Dog>();
ICollection<Mammal> mammalList = dogList; //illegal but for the sake of showing, do it
mammalList.Add(new Cat());

您 mammalList （这实际上是一个 dogList ）现在会那么包含猫。

Your mammalList (which is actually a dogList) would now then contain a Cat.

的IEnumerable 是协变的，因为你不能添加将其......你只能从中读取 - 这，反过来，保留类型安全

IEnumerable is covariant because you cannot Add to it... you can only read from it -- which, in turn, preserves type-safety.

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