如何转换IDictionary <TKey, List<TValue> &gt;到IDictionary <TKey, IEnumerable<TValue> &gt;？

Question

I have a function that return a IDictionary<TKey, List<TValue> > .
I have another function that takes a IDictionary<TKey, IEnumerable<TValue> > .

I need to pass the return of the first function to the second function.
The compiler doesn't want to implicitlty convert the first into the second. So how could I convert the first into the second in O(1)?

I could always write a conversion function

public static IDictionary<TKey, IEnumerable<TValue>> ToIEnumerable<TKey, TValue>(this IDictionary<TKey, List<TValue>> source)
{
    return source.ToDictionary(s => s.Key, s => s.Value.AsEnumerable());
}

But I'm pretty sure this is not O(1), more like O(log n).

A related question here : Nested Interfaces: Cast IDictionary< TKey, IList< TValue > > to IDictionary< TKey, IEnumerable < TValue> >?

Answer 1

Consider your second function, the one that

 takes a IDictionary<TKey, IEnumerable<TValue> >

Now, if you give me a IDictionary<TKey, IEnumerable<TValue>> , one of the things I should be able to do is set the value of one of the keys to a new TValue[] , right? After all, a TValue[] is an IEnumerable<TValue> , so it meets the constraint.

However, if somehow what you'd given me was (underneath the interface) a IDictionary<TKey, List<TValue>> , I definitely shouldn't be allowed to set the value of a key to a TValue[] , since a TValue[] is not a List<TValue> !

This is exactly the problem in the question you have linked, of course.

Now, if you are happy to only ever read the parameter I pass you, you have a potential solution in moving from dictionaries to ILookup<TKey, TValue> , which is a bit like a read-only dictionary from a key to a list of values. If your methods respectively return and accept an ILookup<TKey, TValue> there's of course no problem.