这是为什么投不可能的？ [英] Why is this cast not possible?

问题描述

interface IFolderOrItem<TFolderOrItem> where TFolderOrItem : FolderOrItem {}

abstract class FolderOrItem {}

class Folder : FolderOrItem {}

abstract class Item : FolderOrItem {}

class Document : Item {}

现在我想做某事是这样的：

now i'm trying to do sth like this:

class Something
{
    IFolderItemOrItem<Item> SelectedItem { get; set; }
    void SomeMagicMethod()
    {
        this.SelectedItem = (IFolderOrItem<Item>)GetMagicDocument();
        // bad bad bad ... ??
    }
    IFolderOrItem<Document> GetMagicDocument()
    {
        return someMagicDocument; // which is of type IFolderOrItem<Document>
    }
}

是否有可能得到这个工作？

is there any possibility to get this working?

推荐答案

如果我正确地读它...那么问题是，仅仅因为富：酒吧，即做的不可以意味着 ISomething＆LT;富＆GT; ：ISomething＆LT;酒吧GT; ...

If I read it correctly... then the problem is that just because Foo : Bar, that does not mean that ISomething<Foo> : ISomething<Bar>...

在一些情况下，方差在C＃4.0可能是一种选择。另外，有时有事情，你可以用普通的方法做（不知道它会帮助在这里，虽然）。

In some cases, variance in C# 4.0 may be an option. Alternatively, there are sometimes things you can do with generic methods (not sure it will help here, though).

---

最近，你可以做C（下方）＃3.0可能是一个非通用基本接口：

The closest you can do in C# 3.0 (and below) is probably a non-generic base interface:

interface IFolderOrItem {}
interface IFolderOrItem<TFolderOrItem> : IFolderOrItem
    where TFolderOrItem : FolderOrItem { }

常用，基本的界面会有，例如<​​code>键入的ItemType {获得;} 来表示所考虑的真正类型。然后用法：

commonly, the base-interface would have, for example, a Type ItemType {get;} to indicate the real type under consideration. Then usage:

IFolderOrItem SelectedItem { get; set; }
...
public void SomeMagicMethod()
{
    this.SelectedItem = GetMagicDocument(); // no cast needed
    // not **so** bad
}

---

---

从规范，这涉及到§25.5.6（ECMA 334第4版）：

From the spec, this relates to §25.5.6 (ECMA 334 v4):

25.5.6转换

构造类型遵循相同的转换规则（§13）
  因为这样做非泛型类型。当应用
  这些规则，基类和
  构造类型的接口应
  如§25.5.3所描述的决定。

Constructed types follow the same conversion rules (§13) as do non-generic types. When applying these rules, the base classes and interfaces of constructed types shall be determined as described in §25.5.3.

之间不存在特别的转换
  比其他构造引用类型
  那些§13描述。尤其是，
  不像数组类型，构造
  引用类型不允许
  合变转换（§19.5）。这个
  是指一个类型列表＆LT; B＆GT; 无
  转换（隐式或
  明确），以列表＆LT; A＆GT; 即使 B 是
  从 A衍生。同样地，无
  转化率从存在列表＆LT; B＆GT; 来
  列表与LT;对象＆gt;

No special conversions exist between constructed reference types other than those described in §13. In particular, unlike array types, constructed reference types do not permit co-variant conversions (§19.5). This means that a type List<B> has no conversion (either implicit or explicit) to List<A> even if B is derived from A. Likewise, no conversion exists from List<B> to List<object>.

[注：为理由
  这很简单：如果一个转换
  列表与LT; A＆GT; 是允许的，那么很显然，
  可以键入 A 的值保存到
  列表。然而，这将破坏
  不变，每一个对象
  B＆GT;键入名单，LT的列表始终是一个值
  类型 B ，否则意外故障
  分配到时，会发生
  集合类。注完]

[Note: The rationale for this is simple: if a conversion to List<A> is permitted, then apparently, one can store values of type A into the list. However, this would break the invariant that every object in a list of type List<B> is always a value of type B, or else unexpected failures can occur when assigning into collection classes. end note]

这同样适用于接口。这会改变的位的在C＃4.0，但只在某些情况下。

The same applies to interfaces. This changes a bit in C# 4.0, but only in some cases.

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