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c#linq
Paul
Paul

Reputation: 3954

Flatten a C# Dictionary of Lists with Linq

I have a Dictionary in C#: 

Dictionary<string, List<string>>

How can I use Linq to flatten this into one List<string> that contains all of the lists in the Dictionary?

Thanks!

Upvotes: 61

Views: 51759

Answers (5)

Jon Skeet
Jon Skeet

Reputation: 1500385

Very easily:

var list = dictionary.Values              // To get just the List<string>s
                     .SelectMany(x => x)  // Flatten
                     .ToList();           // Listify

Here the SelectMany call takes a sequence of inputs (the lists which make the values of the dictionary) and projects each single input into another sequence of outputs - in this case "the elements of the list". It then flattens that sequence of sequences into a single sequence.

Upvotes: 114

Keith Nicholas
Keith Nicholas

Reputation: 44288

as a query

var flattened = from p in dictionary
                from s in p.Value
                select s;

or as methods...

var flattened = dictionary.SelectMany(p => p.Value);

I like this over what others have done as I'm passing the whole dictionary into the Linq query rather than just the values.

Upvotes: 12

Julien May
Julien May

Reputation: 2051

Assuming you have an instance called dict:

dict.SelectMany(pair => pair.Value.Select(str => str));

Upvotes: 2

zmbq
zmbq

Reputation: 39013

You should try something like this:

dict.Values.Aggregate(new List<String>(), (a, b) => a.Concat(b));

Upvotes: 1

porges
porges

Reputation: 30580

SelectMany is the easiest way to flatten things:

Dictionary.Values.SelectMany(x => x).ToList()

Upvotes: 8

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