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....

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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.

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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?

Pitarys

I think your question might have been fine-tuned to say "I just want a list of all the elements in the lists that are contained in a dictionary's keys". A list of all the values in the dictionary (what you asked for) is exactly what you were getting: a list of lists. – Rinee

Answer 1

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).

Lampoon

Cardinality mismatch. It's a dictionary of lists, if you want a single dimensional sequence of all elements you need to use SelectMany it flattens the collection of lists into a single sequence. – Lampoon

Answer 2

Noticed a lot of answer were quite old.

This will also work:

using System.Linq;

dict.Values.ToList();

Solarium

Very simple, the answer should be changed to this. – Gesticulation

@rolls No, because dict.Values.ToList() is not the same as dict.Values.SelectMany(x => x) the latter results in a lists of lists while SelectMany flattens and concatenates the resulting lists into a single sequence. – Lampoon

I think you need to import some library to be able to use tolist() – Closefitting

@J.Chang System.Linq – Solarium

I have no idea why this has so many upvotes. This doesn't solve the question asked at all as @JohnLeidegren mentioned...it doesn't "flatten" the dictionary values – Cheree

The title is just misleading. I also just wanted to get the list and for that this works well. – Header

Is there some way we can change this to be the selected answer? – Siderolite

you should change the title of the question to match the question, not change the answer – Teufert

Answer 3

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();

Aberdare

Ah yes, I thought the Values property wasn't accessible through the IDictionary`2 interface. – Lampoon

Answer 4

dictionary.values.toList();

if You want to get Sum just do

myDictionary.values.sum();

Subsellium

Answer 5

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.

Spectre