如何选择IEnumerable <T> 而不是列表 <T> 来自ICollection <T> ？

Question

I have the following lines that work as supposed to.

Dictionary<Guid, List<T>> dictionary = source.ToDictionary(
  element => element.Id,
  element => element.Ts.ToList());

However, I'd like the first line to use IEnumerable , which creates an error of type mismatch.

Dictionary<Guid, IEnumerable<T>> dictionary = source.ToDictionary( ... );

Using ToArray instead of ToList won't help me (getting the same error, plus I've learned not to use arrays in C# unless really, really required) And as far I can tell, the type of Ts is ICollection .

Is it possible to convert the whole shabang the way I wish without going hardcore and using casting?

Answer 1

That's because Dictionary<TKey, TValue> is not covariant. So you can't pick Dictionary<string, List<T>> and cast it to Dictionary<string, IEnumerable<T>> even though List<T> implements IEnumerable<T> . Why? Consider following code:

Dictionary<string, IEnumerable<int>> myDict = new Dictionary<string, List<int>>();
myDict.Add("key", new HashSet<int>());

From compiler perspective that would be totally ok, because HashSet<T> implements IEnumerable<T> . But the dictionary is expecting the value to be List<T> !

You can do a cast in ToDictionary method:

Dictionary<Guid, IEnumerable<T>> dictionary = source.ToDictionary(
  element => element.Id,
  element => (IEnumerable<T>)element.Ts.ToList());

or provide generic type parameters to ToDictionary call:

Dictionary<Guid, IEnumerable<T>> dictionary = source.ToDictionary<Guid, IEnumerable<T>>(
  element => element.Id,
  element => element.Ts.ToList());

Answer 2

Use AsEnumerable() , which is a handy Linq method that just returns the source cast to IEnumerable<T> :

Dictionary<Guid, IEnumerable<T>> dictionary = source.ToDictionary(
  element => element.Id,
  element => element.Ts.AsEnumerable());

you could also try casting:

Dictionary<Guid, IEnumerable<T>> dictionary = source.ToDictionary(
  element => element.Id,
  element => (IEnumerable<T>)(element.Ts));

Answer 3

If you want to avoid casting, you can use the AsEnumerable extension method, which would return the correct type, but also hide the original instance/implementation behind an anonymous enumerable.

I would personally just use element => element.Ts as IEnumerable<T> though, to avoid calling extra method only to satisfy the limited generics inference.