Use LINQ to select distinct properties in Lists of Lists

Question

I want to use LINQ to select a unique list of strings, stored as a List, inside an object. This object is itself stored in a List inside another object. It's hard to explain, here is an example:

public class Master { public List<DataCollection> DateCollection { get; set; } public Master() { this.DateCollection = new List<DataCollection>(); } } public class DataCollection { public List<Data> Data { get; set; } public DataCollection() { this.Data = new List<Data>(); } } public class Data { public string Value{ get; set; } public Data() { } }

Using the Master class, I want to get a list of unique Value strings in the Data class. I have tried the following:

List<string> unique = master.Select(x => x.DataCollection.Select(y => y.Value)).Distinct().ToList();

Can somebody show me how it's done?

So you want the distinct Data of all your DataCollection's data stapled together? — ryanyuyu
– ryanyuyu, Commented Apr 16, 2015 at 15:52
make distinct go in inner parantheses such as master.select(x=>x.DataCollection.Select(y=>y.value).Distinct()) . Your metohd right now gets distinct master not distinct inner values — user3094087
– user3094087, Commented Apr 16, 2015 at 15:52

Alex · Accepted Answer · 2015-04-16 16:02:56Z

You can do that like this, directly using the public DateCollection member:

var unique = master.DateCollection .SelectMany(x => x.Data.Select(d => d.Value)) .Distinct() .ToList();

The key being SelectMany to "flatten" the selection.

Matthias Meid · Accepted Answer · 2015-04-16 15:53:35Z

SelectMany projects a list of lists into a single list:

List<string> unique = master .SelectMany(x => x.DataCollection.Select(y => y.Value)) .Distinct() .ToList();

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