嵌套接口：Cast IDictionary< TKey，IList< TValue>到IDictionary< TKey，IEnumerable< TValue>>？ [英] Nested Interfaces: Cast IDictionary<TKey, IList<TValue>> to IDictionary<TKey, IEnumerable<TValue>>?

问题描述

我认为将 IDictionary 对象转换为 IDictionary< TKey，IEnumerable& TValue>> ，但

I would think it's fairly straightforward to cast an IDictionary<TKey, IList<TValue>> object to an IDictionary<TKey, IEnumerable<TValue>>, but

var val = (IDictionary<TKey, IEnumerable<TValue>>)Value;

引发 System.InvalidCastException / p>

throws a System.InvalidCastException, and

var val = Value as IDictionary<TKey, IEnumerable<TValue>>;

使 val

推荐答案

我认为这是很容易的。 IDictionary > 对 IDictionary< TKey，IEnumerable< TValue>< / code& / p>


I would think it's fairly straightforward to cast an IDictionary<TKey, IList<TValue>> object to an IDictionary<TKey, IEnumerable<TValue>>

绝对不是。它不会是类型安全的。下面是一个为什么不是的例子：

Absolutely not. It wouldn't be type-safe. Here's an example of why not:

// This is fine...
IDictionary<string, IList<int>> dictionary = new Dictionary<string, IList<int>>();

// Suppose this were valid...
IDictionary<string, IEnumerable<int>> badDictionary = dictionary;

// LinkedList<T> doesn't implement IList<T>
badDictionary["foo"] = new LinkedList<int>();

// What should happen now?
IList<int> bang = dictionary["foo"];

正如你所看到的，这将导致问题 - 我们试图得到一个<$当我们期望所有的值都实现 IList< int> 时，c $ c> LinkedList< int> 泛型的要点是类型安全 - 那么你期望哪一行会失败？第一，第三和第四行看起来非常清楚对我有用 - 所以第二个是只有一个可能无法编译，它... ...

As you can see, that's going to cause problems - we'd be trying to get a LinkedList<int> out when we expect all the values to implement IList<int>. The point of generics is to be type-safe - so which line would you expect to fail? The first, third and fourth lines look pretty clearly valid to me - so the second one is the only one which can fail to compile, and it does...

现在在一些例子中，它可以安全地完成。例如，您可以将 IEnumerable 转换为 IEnumerable< object> c $ c> IEnumerable< T> 只在输出位置使用 T 。

Now in some cases, it can be done safely. For example, you can convert (in C# 4) from IEnumerable<string> to IEnumerable<object> because IEnumerable<T> only uses T in "output" positions.

有关详情，请参见 MSDN 。

编辑：只是为了澄清 - 很容易用现有键/值对的副本创建一个新的字典，例如：使用连结：

Just to clarify - it's easy to create a new dictionary with a copy of the existing key/value pairs, e.g. using link:

var copy = original.ToDictionary<TKey, IEnumerable<TValue>>(pair => pair.Key,
                                                            pair => pair.Value);

您只需要知道您现在有两个单独的字典。

You just need to be aware that you now have two separate dictionaries.

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