C#の概念 LINQ（クエリ結果をメモリに格納する）

　ToList()系。

成果物

• github

情報源

• https://docs.microsoft.com/ja-jp/dotnet/csharp/linq/store-the-results-of-a-query-in-memory

コード

ToArray, ToList

class Main {
    public void Run() {
        TestArrayAndList();
    }
    private void TestArrayAndList() {
        var list = new List<int>() { 5, 1, 4, 2, 3 };
        var q = from v in list
                where 1 == (v % 2)
                select v;
        Console.WriteLine($"ToArray: {string.Join(",", list.ToArray())}");
        Console.WriteLine($"ToList: {string.Join(",", list.ToList())}");
    }
}

ToArray: 5,1,4,2,3
ToList: 5,1,4,2,3

ToDictionary

　まずは辞書の元となるデータソースを生成する。

class Human {
    public string Name { get; }
    public int Age { get; }
    public Human(string name, int age) => (Name, Age) = (name, age);
}

    private List<Human> CreateHumans() {
        return new List<Human> {
            new Human("A", 0),
            new Human("B", 1),
            new Human("C", 2),
            new Human("D", 3),
            new Human("E", 4),
            new Human("F", 5),
        };
    }
    private IEnumerable<Human> Query(in List<Human> humans) {
        return from h in humans
               where 1 == (h.Age % 2)
               select h;
    }

　クエリ結果を辞書に変換する。

class Main {
    public void Run() {
        TestDictionary();
    }

    private void TestDictionary() {
        List<Human> humans = CreateHumans();
        ShowDictionary(Query(in humans).ToDictionary(h => h.Name));
    }
    private void ShowDictionary(in IDictionary<string, Human> dict) {
        foreach (KeyValuePair<string, Human> kv in dict) {
            Console.WriteLine($"Key={kv.Key} Value=Name:{kv.Value.Name}, Age:{kv.Value.Age}");
        }
    }

Key=B Value=Name:B, Age:1
Key=D Value=Name:D, Age:3
Key=F Value=Name:F, Age:5

ToLookup

    private void TestLookup() {
        List<Human> humans = CreateHumans();
        ShowLookup(Query(in humans).ToLookup(h => h.Name));
    }
    private void ShowLookup(in ILookup<string, Human> lookup) {
        foreach (var g in lookup) {
            Console.WriteLine($"Key={g.Key}");
            foreach (Human h in g) {
                Console.WriteLine($"Name={h.Name}, Age={h.Age}");
            }
        }
    }

Key=B
Name=B, Age=1
Key=D
Name=D, Age=3
Key=F
Name=F, Age=5

　ちなみに、以下だと実行時エラーになった。ILookupにキャストできないっぽい。よくわからん。

    private void ShowLookup(in ILookup<string, Human> lookup) {
        foreach (ILookup<string, Human> g in lookup) {
            Console.WriteLine($"Key={g.Key}");
            foreach (Human h in g) {
                Console.WriteLine($"Name={h.Name}, Age={h.Age}");
            }
        }
    }

System.InvalidCastException: Specified cast is not valid.

対象環境

• 2019-11-30
• Raspbierry pi 3 Model B+
• Raspbian stretch 9.0 2018-11-13 ※
• bash 4.4.12(1)-release ※
• SQLite 3.29.0 ※
• C# dotnet 3.0.100 ※

$ uname -a
Linux raspberrypi 4.19.42-v7+ #1218 SMP Tue May 14 00:48:17 BST 2019 armv7l GNU/Linux