C＃类型转换：显式转换存在但抛出一个转换错误？

问题描述

我了解到 HashSet 实现了 IEnumerable 接口。因此，可以将 HashSet 对象隐式转换为 IEnumerable ：

I learned that HashSet implements the IEnumerable interface. Thus, it is possible to implicitly cast a HashSet object into IEnumerable:

HashSet<T> foo = new HashSet<T>();
IEnumerable<T> foo2 = foo; // Implicit cast, everything fine.

这也适用于嵌套泛型类型：

This works for nested generic types, too:

HashSet<HashSet<T>> dong = new HashSet<HashSet<T>>();
IEnumerable<IEnumerable<T>> dong2 = dong; // Implicit cast, everything fine.

至少这就是我的想法。但是如果我做一个字典，我遇到一个问题：

At least that's what I thought. But if I make a Dictionary, I run into a problem:

IDictionary<T, HashSet<T>> bar = new Dictionary<T, HashSet<T>>();
IDictionary<T, IEnumerable<T>> bar2 = bar; // compile error

最后一行给我以下编译错误（Visual Studio 2015）： p>

The last line gives me the following compile error (Visual Studio 2015):

System.Collections.Generic .IDictionary< T，System.Collections.Generic.HashSet< T> 到
System.Collections.Generic.IDictionary< T，System.Collections.Generic

System.Collections.Generic.IDictionary<T, System.Collections.Generic.HashSet<T>> to System.Collections.Generic.IDictionary<T, System.Collections.Generic.IEnumerable<T>>.

存在显式转换（您是否缺少演员？）

An explicit conversion exists (are you missing a cast?)

但是，如果我通过写作执行演员

But if I do the cast by writing

IDictionary<T, IEnumerable<T>> bar2 = (IDictionary<T, IEnumerable<T>>) bar;

然后我在运行时得到一个无效的转换异常。

then I get an invalid cast exception at runtime.

两个问题：

• 我该如何解决？是唯一的方法来迭代密钥，并逐一建立一个新的字典？


• 为什么我首先得到这个问题，即使 HashSet 实现 IEnumerable 界面？

• How do I solve this? Is the only way to iterate over the keys and build up a new dictionary bit by bit?
• Why do I get this problem in the first place, even thoughHashSet does implement the IEnumerable interface?

推荐答案

它不起作用的原因是 IDictionary< TKey，TValue> 中的值不是（同样的原因也不是关键）。如果它被允许，那么这个代码将被编译，但是有会导致一个异常：

The reason it doesn't work is that the value in IDictionary<TKey, TValue> is not co-variant (and nor is the key, for the same reasons). If it were allowed to be, then this code would compile, but has to result in an exception:

IDictionary<T, HashSet<T>> foo = new Dictionary<T, HashSet<T>>();
IDictionary<T, IEnumerable<T>> bar = foo;
foo.Add(key, new List<T>());

你会认为添加一个列表< T> 将工作，因为它将编译，因为值类型应该是 IEnumerable< T> 。但是，由于实际值类型是 HashSet 。

You'd think adding a List<T> would work, as it would compile given the value type is supposedly IEnumerable<T>. It can't succeed, though, as the actual value type is HashSet<T>.

所以，是的，唯一的方法是创建一个新的字典。

So, yes: the only way is to create a new dictionary.

var bar = foo.ToDictionary(x => x.Key, x => x.Value.AsEnumerable());

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