C＃类型转换：显式类型转换存在，但会抛出一个转换错误？ [英] C# type conversion: Explicit cast exists but throws a conversion error?

问题描述

我了解到， HashSet的实施的IEnumerable 接口。因此，有可能隐含蒙上了 HashSet的对象为的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.

这适用于嵌套泛型类型，也：

This works for nested generic types, too:

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

目前至少这是我的想法。但是，如果我做一个词典，我碰到一个问题：

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

最后一行给我下面的编译错误（Visual Studio的2015年）：

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

无法隐式转换类型

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

这是显式转换存在（是否缺少强制转换？）

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

但是，如果我写做演员

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

然后我得到在运行时无效转换异常。

then I get an invalid cast exception at runtime.

两个问题：

• 我如何解决这个问题？是遍历键和滴滴？建立一个新的字典位的唯一方法


• 为什么我会在第一时间这个问题，即使的HashSet 确实实现了的IEnumerable 接口？

• 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

它不工作的原因是，在的IDictionary<的价值; TKEY的，TValue> 不是的共变的（并且也不是关键，出于同样的原因）。如果它被允许存在，那么这段代码编译，但的具有的导致异常：

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>());

您会想加入列表< T> 会的工作，因为这将编译给定的值类型是所谓的IEnumerable< T> 。它不能成功，不过，作为的实际的值类型为的HashSet< T>

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());

这篇关于C＃类型转换：显式类型转换存在，但会抛出一个转换错误？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！