协方差在C＃泛型类 [英] Covariance in C# generic class

问题描述

C＃4.0 .NET 4.5的Silverlight 5
这似乎不可思议，我不能找到解决方案，因此需要一些帮助，请。

C# 4.0 .NET 4.5 Silverlight 5 It seems weird that I cant find the solution so need some help please.

我有基类基地和派生类儿童：基础。我有泛型类型做具体工作1 EF实体助手也是辅助类，其中T：EntityObject

I have base class Base and derived class Child : Base. I have also helper class which has generic type to do specific work one EF entities Helper where T : EntityObject.

孩子不具体工作与特定的实体myEntity所。EntityObject

Child does specific work with a specific entity MyEntity : EntityObject.

所以，我想：

public class Base
{
    protected Helper<EntityObject> helper;
}
public class Child : Base
{
    public Child()
    {
        helper = new Helper<MyEntity>();
    }
}

我希望有更多的派生类必须了解更多具体的泛型参数，我想这就是协方差...但还是不行...

I would expect that more derived class must know about more specific generic parameter and I think that's what covariance for... But that doesn't work...

什么是正确的方式来设计类这样？

What's the 'correct' way to design class like that?

编辑：对不起，我没能100％的清楚为什么不能我实现我所需要的。

sorry I didn't make it 100% clear why cant I achieve what I need.

。与一般的基本解决方案不起作用，因为基本的用户不知道的T类。想象一下：

a. Solution with generic Base doesn't work because user of the Base doesn't know the T type. Imagine:

public class User
{
    private Base<T> base; // this will not compile.
    public User(TypeEnum t)
    {
        if(t == TypeEnum.MyEntity) base = new Child();
...

乙。与接口解决方案不起作用，因为辅助使用牛逼无处不在（这是它的正确的目的是什么？）。想象一下，它有法

b. Solution with Interface doesn't work because helper uses T everywhere (it is its purpose right?). Imagine it has method

public IEnumerable<T> Process(IEnumerable<T> items) { return items; }

我如何把它在不知道什么牛逼接口

How do I bring it up in the interface which doesn't know anything about T

推荐答案

如果富：酒吧，这并不意味着部分<富> ：部分<酒吧GT; 。有这样做你想要什么有两种方式。首先是制造基地型通用这样：

If Foo : Bar, that doesn't mean that Some<Foo> : Some<Bar>. There are two ways of doing what you want. The first is to make the base-type generic such that:

Base<T> where T : EntityObject {
    protected Helper<T> helper;
}
Child : Base<MyEntity> {...}

第二个是在基础类型为使用非通用接口，即有

The second is to use a non-generic interface at the base-type, i.e. have

Base {
    protected IHelper helper;
}
Child : Base {...}

其中后者情况下，助手< T> ：IHelper ，对于一些非通用 IHelper 待定义

where in the latter case, Helper<T> : IHelper, for some non-generic IHelper to-be-defined.

作为一个侧面说明，你的可能的发现很容易向下传递值在构造函数，而不是使用保护字段。

As a side-note, you might find it easier to pass the value down in the constructor rather than using a protected field.

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