Different ways of using SelectMany()

Question

I'd like to know how to use SelectMany(). It seems to take so many arguments and from my own research I noticed that SelectMany() might be the 'father' of all other select operations.

Answer 1

Select many allows you to select a property from your query source that is an IEnumerable<T> collection, but instead of returning a collection of collections (IEnumerable<IEnumerable<T>>) it will flatten the collections into a single collection.

Here's an example that you can run to demonstrate the differences between Select and SelectMany:

//set up some data for our example
var tuple1 = new { Name = "Tuple1", Values = new int [] { 1, 2, 3 } };
var tuple2 = new { Name = "Tuple2", Values = new int [] { 4, 5, 6 } };
var tuple3 = new { Name = "Tuple3", Values = new int [] { 7, 8, 9 } };

//put the tuples into a collection
var tuples = new [] { tuple1, tuple2, tuple3 };

//"tupleValues" is an IEnumerable<IEnumerable<int>> that contains { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } }
var tupleValues = tuples.Select(t => t.Values);

//"tupleSelectManyValues" is an IEnumerable<int> that contains { 1, 2, 3, 4, 5, 6, 7, 8, 9 }
var tupleSelectManyValues = tuples.SelectMany(t => t.Values);

By using SelectMany you make it easier to query values within child collections.

Answer 2

There are several overloads to SelectMany. One of them allows you to keep track of any relationship between parent and children while traversing the hierarchy.

Example: suppose you have the following structure: League -> Teams -> Player

You can easily return a flat collection of players. However you may loose any reference to the team a player is part of.

Fortunately there is an overload for such purpose:

var teamsAndTheirLeagues = 
         from helper in leagues.SelectMany
               ( l => l.Teams
                 , ( league, team ) => new { league, team } )
                      where helper.team.Players.Count > 2 
                           && helper.league.Teams.Count < 10
                           select new 
                                  { LeagueID = helper.league.ID
                                    , Team = helper.team 
                                   };

The previous example is taken from Dan's IK blog:

http://blogs.interknowlogy.com/2008/10/10/use-linqs-selectmany-method-to-flatten-collections/

I strongly recommend you take a look at it.