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!
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5 Answers
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.
6 Comments
Paul
Cool. How does SelectMany differ from Select?
Jon Hanna
@Paul I'm so tempted to just comment saying, "it selects many", but actually that's pretty much it. If you have an enumerable (or queryable) object that when enumerated returns yet more enumerable objects, such as a list of lists or an array of hash-sets, etc, it selects the elements of those objects. So in this where values is an
ICollection<List<string>> it returns an IEnumerable<string>.
Jon Skeet
@Default: Yup - see codeblog.jonskeet.uk/2010/12/27/…
philologon
@JonHanna Best to-the-point explanation of SelectMany I have ever seen. Not only brief, but something I can remember.
Luiz Bicalho
The problem with this is that we los the keys, I imagined something like a key/value with the keys duplicated
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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.
Comments
SelectMany is the easiest way to flatten things:
Dictionary.Values.SelectMany(x => x).ToList()
Comments
Assuming you have an instance called dict:
dict.SelectMany(pair => pair.Value.Select(str => str));
Comments
You should try something like this:
dict.Values.Aggregate(new List<String>(), (a, b) => a.Concat(b));
