[C#] Select와 SelectMany의 차이점

SelectMany목록 목록을 반환하는 쿼리를 병합합니다. 예를 들어

public class PhoneNumber
{
    public string Number { get; set; }
}

public class Person
{
    public IEnumerable<PhoneNumber> PhoneNumbers { get; set; }
    public string Name { get; set; }
}

IEnumerable<Person> people = new List<Person>();

// Select gets a list of lists of phone numbers
IEnumerable<IEnumerable<PhoneNumber>> phoneLists = people.Select(p => p.PhoneNumbers);

// SelectMany flattens it to just a list of phone numbers.
IEnumerable<PhoneNumber> phoneNumbers = people.SelectMany(p => p.PhoneNumbers);

// And to include data from the parent in the result: 
// pass an expression to the second parameter (resultSelector) in the overload:
var directory = people
   .SelectMany(p => p.PhoneNumbers,
               (parent, child) => new { parent.Name, child.Number });

.NET Fiddle의 라이브 데모

답변

Set A={a,b,c}
Set B={x,y}

{ (x,a) , (x,b) , (x,c) , (y,a) , (y,b) , (y,c) }

List<string> animals = new List<string>() { "cat", "dog", "donkey" };
List<int> number = new List<int>() { 10, 20 };

var mix = number.SelectMany(num => animals, (n, a) => new { n, a });

{(10,cat), (10,dog), (10,donkey), (20,cat), (20,dog), (20,donkey)}

var players = db.SoccerTeams.Where(c => c.Country == "Spain")
                            .SelectMany(c => c.players);

foreach(var player in players)
{
    Console.WriteLine(player.LastName);
}

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
                                   };

var orders = customers
             .Where(c => c.CustomerName == "Acme")
             .SelectMany(c => c.Orders);