List to Dictionary<Key, List<Value>> - C#

Question

I have a List and MyClass is:

public class MyClass
{
    public bool Selected { get; set; }
    public Guid NoticeID { get; set; }
    public Guid TypeID { get; set; }
}

My question is, how do i convert this list into a Dictionary<Guid, List<Guid>>, where the dictionary key is the GUID from the TypeID property, and the value is a list of all the NoticeID values corresponding to that TypeID. I have tried like so:

list.GroupBy(p => p.TypeID).ToDictionary(p => p.Key, p => p.ToList())

but this returns a Dictionary <Guid, List<MyClass>>, and I want a Dictionary<Guid, List<Guid>>.

Answer 1

Well, when you group you can specify the value you want for each element of the group:

var dictionary = list.GroupBy(p => p.TypeID, p => p.NoticeID)
                     .ToDictionary(p => p.Key, p => p.ToList());

However, I would strongly consider using a lookup instead of a dictionary:

var lookup = list.ToLookup(p => p.TypeID, p => p.NoticeID);

Lookups are much cleaner in general:

• They're immutable, whereas your approach ends up with lists which can be modified
• They express in the type system exactly what you're trying to express (one key to multiple values)
• They make looking keys up easier by returning an empty sequence of values for missing keys, rather than throwing an exception