C＃泛型铸造（协方差和逆变？） [英] C# Casting generics (covariance and contravariance?)

问题描述

我需要一些建议/在这方面的帮助，我不能看到木从树上了。

I need some advice/help on this, I can't see the wood from the trees any more.

这是一个简单的一系列类实现使用泛型一些接口。

It's a straight forward series of classes implementing some interfaces using generics.

然后我想投的具体类型，例如：

Then I'm trying to cast the concrete types for example:

MyGenericObject<SomeObject> _obj;

IMyGenericObject<ISomeObject> _genObj = (IMyGenericObject<ISomeObject>)_obj;

//无效的投

// Invalid cast

我读过有关协变和逆变，但也不是太清楚一些文章为什么这是不可能的，或者如何避开它？

I've read some articles about covariance and contravariance but not too clear why this wouldn't be possible, or how to get round it?

所以，在这个例子中：

public interface IMyObject<in T> where T : IBaseObject
{
    T Activity { get; set; }
}

是行不通的......

wouldn't work...

......，因为，你不能获取和设置活动属性。

....because, you can't get and set the Activity property.

在这个例子中，我需要做的：

In this example, I needed to do:

public interface IMyObject<out T> where T : IBaseObject
    {
        T Activity { get; }
    }

希望帮助别人，并感谢所有帮助！

hope that helps someone, and thanks to all for help!

推荐答案

您只能做，如果你声明的接口为具有协变（退出）的参数。你只能这样做，如果该参数是协变使用。

You can only do that if you declare the interface as having a covariant (out) parameter. You can only do that if the parameter is used covariantly.

例如，如果接口 IMyGenericObject＆LT; T＆GT; 有服用 T 参数的方法，这prevents你声明参数作为协变。相反，如果有，返回的方法的 T ，即prevents你声明参数作为逆变。

For example, if the interface IMyGenericObject<T> has a method taking a T parameter, this prevents you from declaring the parameter as covariant. Conversely, if there is a method that returns a T, that prevents you from declaring the parameter as contravariant.

修改

在回答关于SLaks的回答您的意见，我很想重复一切埃里克利珀曾经写了合作和逆变。请参阅<一href="http://blogs.msdn.com/b/ericlippert/archive/tags/Covariance+and+Contravariance/">http://blogs.msdn.com/b/ericlippert/archive/tags/Covariance+and+Contravariance/也是自己的答案，在SO（最近 http://stackoverflow.com/a/8380213/385844 ）

In response to your comment on SLaks's answer, I'm tempted to repeat everything Eric Lippert has ever written on co- and contravariance. See http://blogs.msdn.com/b/ericlippert/archive/tags/Covariance+and+Contravariance/ and also his answers in SO (most recently http://stackoverflow.com/a/8380213/385844)

要总结一下：

您无法施展的IList＆LT;字符串＆GT; 到的IList＆LT;对象＆gt; ，因为它是合法的传递的FileInfo 到的IList＆LT;对象＆gt; ，但它是不合法的将它传递给一个的IList＆LT ;字符串＆GT;

You can't cast IList<string> to IList<object> because it's legal to pass a FileInfo to an IList<object>, but it is not legal to pass it to an IList<string>.

您不能施放一个的IList＆LT;对象＆gt; 到的IList＆LT;字符串＆GT; ，因为它是合法的字符串＆GT;从的IList＆LT检索项目并将其分配给一个字符串引用，但一个的IList＆LT;对象＆gt; 可能含有一个FileInfo，这是不能被分配​​给一个字符串引用

You can't cast an IList<object> to an IList<string>, because it's legal to retrieve an item from an IList<string> and assign it to a string reference, but an IList<object> might contain a FileInfo, which can't be assigned to a string reference.

编辑2

既然你问的意见，它也可以在您的接口分成合作和逆变部分。要继续列表中的例子，你可以有这些接口

Since you asked for advice, it's also possible to split your interfaces into co- and contravariant parts. To continue with the list example, you could have these interfaces

public interface ICovariantList<out T>
{
    T this[int index] { get; }
    //...
}

public interface IContravariantList<in T>
{
    T this[int index] { set; }
    void Add(T item);
    //...
}

public class SomeList<T> : ICovariantList<T>, IContravariantList<T>
{
    //...
}

这可以让你使用类协变或contravariantly，要看具体情况。

This allows you to use the class covariantly or contravariantly, depending on the context.

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