嵌套泛型集合：如何实现从项目到容器的参考？

问题描述

在使用嵌套的泛型集合实现的设计，我偶然发现显然是由C＃造成这些限制的不变的泛型：

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，以下将无法正常工作，显然是由于仿制药的不变性：

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; }
	}
}

（在实际的代码中，无论内蒙古<> 和外<> 实施的ICollection<> 。）

我需要的内蒙古<> 对象有其容器收集，以获得一个参考它的一些数据。

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

您会如何最好使用如上文所述的通用方法实现这些嵌套的集合，？你会如何设置在内蒙古<的参考容器收集;> 类？

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?

干杯！

推荐答案

引入一个（可能是抽象）基类它不依赖于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; }
    }
}

或等待C＃4.0，它引入了CO /禁忌变种的通用接口。

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

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