C#] LINQ - Projection

Linq에서 projection은 지속 사용할 properties만 담긴 새로운 object를 생성하는 작업이다. 

 

projection operations

1. Select

2. SelectMany

 

class Bouquet
{
    public List<string> Flowers { get; set; }
}

static void SelectVsSelectMany()
{
    List<Bouquet> bouquets = new()
    {
        new Bouquet { Flowers = new List<string> { "sunflower", "daisy", "daffodil", "larkspur" }},
        new Bouquet { Flowers = new List<string> { "tulip", "rose", "orchid" }},
        new Bouquet { Flowers = new List<string> { "gladiolis", "lily", "snapdragon", "aster", "protea" }},
        new Bouquet { Flowers = new List<string> { "larkspur", "lilac", "iris", "dahlia" }}
    };

    IEnumerable<List<string>> query1 = bouquets.Select(bq => bq.Flowers);

    IEnumerable<string> query2 = bouquets.SelectMany(bq => bq.Flowers);

    Console.WriteLine("Results by using Select():");
    // Note the extra foreach loop here.
    foreach (IEnumerable<String> collection in query1)
        foreach (string item in collection)
            Console.WriteLine(item);

    Console.WriteLine("\nResults by using SelectMany():");
    foreach (string item in query2)
        Console.WriteLine(item);

    /* This code produces the following output:

       Results by using Select():
        sunflower
        daisy
        daffodil
        larkspur
        tulip
        rose
        orchid
        gladiolis
        lily
        snapdragon
        aster
        protea
        larkspur
        lilac
        iris
        dahlia

       Results by using SelectMany():
        sunflower
        daisy
        daffodil
        larkspur
        tulip
        rose
        orchid
        gladiolis
        lily
        snapdragon
        aster
        protea
        larkspur
        lilac
        iris
        dahlia
    */
}