嵌套的通用集合：如何实现从项到容器的引用？

问题描述

在使用嵌套泛型集合实现设计时，我偶然发现了这些限制显然是由C＃的不变的泛型造成的：

While implementing a design using nested generic collections, I stumbled across those limitations apparently caused by C#'s invariant Generics:

Cannot convert from 'Collection<subtype of T> to 'Collection<T>'

这意味着，以下操作将不起作用，显然是由于泛型的不变性：

That means, the following will not work, apparently due to the invariance of Generics:

class Outer<TInner, TInnerItem> where TInner : Inner<TInnerItem>
{
    public void Add(TInner item)
    {
        item.Outer = this; // ERROR:
            // Cannot implicitly convert from Outer<TInner, TInnerItem>
            // to Outer<Inner<TInnerItem>, TInnerItem>
    }
}

class Inner<TInnerItem> : ICollection<TInnerItem>
{
    Outer<Inner<TInnerItem>, TInnerItem> _outer;

    public Outer<Inner<TInnerItem>, TInnerItem> Outer
    {
        set { _outer = value; }
    }
}

（在实际代码中， Inner<> 和外部<> 实现 ICollection< 。）

(In the actual code, both Inner<> and Outer<> implement ICollection<>.)

我需要 Inner<> 对象来引用其容器集合，以便访问一些数据。

I need the Inner<> objects to have a reference to its container collection in order to access some of its data.

如何实现这些嵌套集合，最好使用上面概述的通用方法？如何在 Inner<> 类中设置对容器集合的引用？

How would you implement these nested collections, preferably using a generic approach as outlined above? How would you set the reference to the container collection in the Inner<> class?

介绍一个（可能是抽象的）基类。

Cheers!

推荐答案

不依赖于TInner可以帮助你：

Introducing a (possibly abstract) base class which is not dependant on TInner may help you:

abstract class OuterBase<TInnerItem>
{
}

class Outer<TInner, TInnerItem> : OuterBase<TInnerItem> where TInner : Inner<TInnerItem>
{
    public void Add(TInner item)
    {
        item.Outer = this; // Compiles
    }
}

class Inner<TInnerItem> : ICollection<TInnerItem>
{
    OuterBase<TInnerItem> _outer;

    public OuterBase<TInnerItem> Outer
    {
        set { _outer = value; }
    }
}



或者等待C＃4.0，反变异通用接口。

Or wait for C# 4.0, which introduces co/contra-variant generic interfaces.

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