它不起作用的原因是 IDictionary< TKey，TValue> 中的值不是共同变体（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());