using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        //整数列 collection から偶数列をもとめる

        List<int> collection = [0,1,2,3,4,5,6,7,8,9];

        List<int> list = [.. from item in collection where item % 2 == 0 select item];

        Console.WriteLine(string.Join(", ", list)); // 0, 2, 4, 6, 8

        //数列をゼロから作るパターン 要拡張メソッド

        List<int> list2 = [.. from x in (0..9).To() where x % 2 == 0 select x];

        Console.WriteLine(string.Join(", ", list2)); 

        //拡張メソッドつかわない（ちょっとリスト内包表記ぽくない）

        List<int> list3 = [.. from x in Enumerable.Range(0,10) where x % 2 == 0 select x];

        Console.WriteLine(string.Join(", ", list3)); 

        //var listF = [.. from x in (0..9).To() where x % 2 == 0 select x]; //There is no target type for the collection expression.

        //(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)

        List<(int, int)> pairs = [ .. from x in (0..3).To() from y in (0..3).To() select (x, y)];

        Console.WriteLine(string.Join(", ", pairs)); 

        List<int> list4 = [.. from item in collection where item > 5 orderby item descending select item];

        Console.WriteLine(string.Join(", ", list4));  //9, 8, 7, 6

    }

}

​

//拡張メソッド

public static class RangeExtensions

{

    public static IEnumerable<int> To(this Range range)

    {

        int start = range.Start.Value;

        int end = range.End.Value;

        return Enumerable.Range(start, end - start);

    }

}