How do I get all the values of a Dictionary<TKey, TValue> as an IList<TValue>?

Question

I have a the following dictionary:

IDictionary<int, IList<MyClass>> myDictionary

and I am wanting to get all the values in the dictionary as an IList....

---

Just to add a bit of a background as to how I've gotten into this situation....

I have a method that gets me a list of MyClass. I then have another method that converts that list into a dictionary where they key is the id for MyClass. Later on...and without access to that original list...I'm needing to obtain the original ungrouped list of MyClass.

---

When I pass myDictionary.Values.ToList() to a method that takes an IList I get a compile error that says that it can't convert from

System.Collections.Generic.List<System.Collections.Generic.IList<MyClass>>

to:

System.Collections.Generic.IList<MyClass>

Now, I can understand that its gone and added each of the groups of IList to the new list as separate elements of the list....but in this instance its not really what I'm after. I just want a list of all the values in the entire dictionary.

How then can I get what I'm after without looping through each of the key values in the dictionary and creating the list I want?

Answer 1

Sorry, I misread the question. You'll need to loop through to combine the groups into a larger list.

Answer 2

Because of how a dictionary (or hash table) is maintained this is what you would do. Internally the implementation contains keys, buckets (for collision handling) and values. You might be able to retrieve the internal value list but you're better of with something like this:

IDictionary<int, IList<MyClass>> dict;
var flattenList = dict.SelectMany( x => x.Value );

It should do the trick ;) SelectMany flattens the result which means that every list gets concatenated into one long sequence (IEnumerable`1).

Answer 3

Values gets a ICollection containing the values of your dictionary. As implied by the definition of your dictionary, it can be defined as a ICollection<IList<MyClass>> collection. So if you really want a IList<IList<MyClass>>, use spacedog's solution.

If what you really want is a flat `IList', then there is no other solution than looping through each value :

IList<MyClass> l=new List<MyClass>();
foreach (IList<MyClass> v in myDictionary.Values)
    l.AddRange(v);

Note that this is so grossly inefficient that you should think again about using a dictionary for what you are trying to achieve.

Answer 4

A variation on John's suggestion:

var flattenedValues = dict.Values.SelectMany(x => x);

If you need them in a list, you can of course call ToList:

var flattenedList = dict.Values.SelectMany(x => x).ToList();