【C#】LINQでわかるデータ操作テクニック総まとめ

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new List<int> { 1, 3, 8, 27, 4, 6 };
        var query = numbers
            .Where(x => x % 2 == 0)    // 偶数を抽出
            .OrderBy(x => x)           // 昇順に並べ替え
            .Select(x => x * 2);       // 2倍に変換
        foreach (var num in query)
        {
            Console.WriteLine(num);
        }
    }
}

8
12
16

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        // IEnumerableの例(メモリ内コレクション)
        IEnumerable<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
        var evenNumbers = numbers.Where(n => n % 2 == 0);
        Console.WriteLine("IEnumerableで偶数を抽出:");
        foreach (var n in evenNumbers)
        {
            Console.WriteLine(n);
        }
        // IQueryableの例(通常はORMなどで使用)
        // ここでは簡単のためIQueryableに変換してみるだけ
        IQueryable<int> queryableNumbers = numbers.AsQueryable();
        var queryableEven = queryableNumbers.Where(n => n % 2 == 0);
        Console.WriteLine("IQueryableで偶数を抽出:");
        foreach (var n in queryableEven)
        {
            Console.WriteLine(n);
        }
    }
}

IEnumerableで偶数を抽出:
2
4
IQueryableで偶数を抽出:
2
4

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new List<int> { 1, 3, 8, 27, 4, 6 };
        var query = from x in numbers
                    where x % 2 == 0
                    orderby x
                    select x * 2;
        foreach (var num in query)
        {
            Console.WriteLine(num);
        }
    }
}

8
12
16

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new List<int> { 1, 3, 8, 27, 4, 6 };
        var query = numbers
            .Where(x => x % 2 == 0)
            .OrderBy(x => x)
            .Select(x => x * 2);
        foreach (var num in query)
        {
            Console.WriteLine(num);
        }
    }
}

8
12
16

Func<int, bool> isEven = delegate(int x) { return x % 2 == 0; };
var numbers = new List<int> { 1, 2, 3, 4, 5 };
var evenNumbers = numbers.Where(isEven);
foreach (var n in evenNumbers)
{
    Console.WriteLine(n);
}

2
4

var numbers = new List<int> { 1, 2, 3, 4, 5 };
var evenNumbers = numbers.Where(x => x % 2 == 0);
foreach (var n in evenNumbers)
{
    Console.WriteLine(n);
}

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        int[] numbers = { 5, 2, 9, 1, 6, 4 };
        var evenSorted = numbers
            .Where(n => n % 2 == 0)
            .OrderBy(n => n);
        foreach (var num in evenSorted)
        {
            Console.WriteLine(num);
        }
    }
}

2
4
6

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var dict = new Dictionary<string, int>
        {
            ["apple"] = 3,
            ["banana"] = 5,
            ["orange"] = 2
        };
        // 値が3以上の要素を抽出
        var filtered = dict.Where(kv => kv.Value >= 3);
        foreach (var kv in filtered)
        {
            Console.WriteLine($"{kv.Key}: {kv.Value}");
        }
    }
}

apple: 3
banana: 5

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var fruits = new[]
        {
            new { Name = "apple", Color = "red" },
            new { Name = "banana", Color = "yellow" },
            new { Name = "cherry", Color = "red" },
            new { Name = "lemon", Color = "yellow" }
        };
        var lookup = fruits.ToLookup(f => f.Color);
        foreach (var group in lookup)
        {
            Console.WriteLine($"Color: {group.Key}");
            foreach (var fruit in group)
            {
                Console.WriteLine($"  {fruit.Name}");
            }
        }
    }
}

Color: red
  apple
  cherry
Color: yellow
  banana
  lemon

using System;
using System.Data;
using System.Linq;
class Program
{
    static void Main()
    {
        var table = new DataTable();
        table.Columns.Add("Id", typeof(int));
        table.Columns.Add("Name", typeof(string));
        table.Columns.Add("Age", typeof(int));
        table.Rows.Add(1, "Alice", 30);
        table.Rows.Add(2, "Bob", 25);
        table.Rows.Add(3, "Charlie", 35);
        var query = table.AsEnumerable()
            .Where(row => row.Field<int>("Age") > 28)
            .OrderBy(row => row.Field<string>("Name"))
            .Select(row => new
            {
                Id = row.Field<int>("Id"),
                Name = row.Field<string>("Name"),
                Age = row.Field<int>("Age")
            });
        foreach (var item in query)
        {
            Console.WriteLine($"{item.Id}: {item.Name} ({item.Age})");
        }
    }
}

1: Alice (30)
3: Charlie (35)

using System;
using System.Linq;
using System.Xml.Linq;
class Program
{
    static void Main()
    {
        string xml = @"
            <Books>
                <Book Id='1' Title='C#入門' Price='3000' />
                <Book Id='2' Title='LINQ実践' Price='3500' />
                <Book Id='3' Title='ASP.NET Core' Price='4000' />
            </Books>";
        var doc = XDocument.Parse(xml);
        var expensiveBooks = doc.Descendants("Book")
            .Where(b => (int)b.Attribute("Price") > 3200)
            .Select(b => new
            {
                Id = (int)b.Attribute("Id"),
                Title = (string)b.Attribute("Title"),
                Price = (int)b.Attribute("Price")
            });
        foreach (var book in expensiveBooks)
        {
            Console.WriteLine($"{book.Id}: {book.Title} - {book.Price}円");
        }
    }
}

2: LINQ実践 - 3500円
3: ASP.NET Core - 4000円

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.Json;
class Program
{
    static void Main()
    {
        string json = @"
        [
            { ""Id"": 1, ""Name"": ""Alice"", ""Age"": 30 },
            { ""Id"": 2, ""Name"": ""Bob"", ""Age"": 25 },
            { ""Id"": 3, ""Name"": ""Charlie"", ""Age"": 35 }
        ]";
        var options = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
        var people = JsonSerializer.Deserialize<List<Person>>(json, options);
        var adults = people
            .Where(p => p.Age >= 30)
            .Select(p => new { p.Name, p.Age });
        foreach (var person in adults)
        {
            Console.WriteLine($"{person.Name} ({person.Age}歳)");
        }
    }
    class Person
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
    }
}

Alice (30歳)
Charlie (35歳)

using System;
using System.Linq;
using Microsoft.EntityFrameworkCore;

class Program
{
    static void Main()
    {
        using var context = new SampleContext();

        // データをシード
        if (!context.Products.Any())
        {
            context.Products.AddRange(
                new Product { Name = "ProductA", Price = 1500 },
                new Product { Name = "ProductB", Price = 2000 },
                new Product { Name = "ProductC", Price = 800 }
            );
            context.SaveChanges();
        }

        // 価格が1000円を超える商品を名前順に取得
        var expensiveProducts = context.Products
            .Where(p => p.Price > 1000)
            .OrderBy(p => p.Name)
            .Select(p => new { p.Name, p.Price });

        // 結果を出力
        foreach (var product in expensiveProducts)
        {
            Console.WriteLine($"{product.Name}: {product.Price}円");
        }
    }
}

public class SampleContext : DbContext
{
    public DbSet<Product> Products { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
        => options.UseInMemoryDatabase("SampleDB");
}

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Price { get; set; }
}

ProductA: 1500円
ProductB: 2000円

using System;
using System.Data.SQLite;
using System.Linq;
using Dapper;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        using var connection = new SQLiteConnection("Data Source=:memory:");
        connection.Open();
        connection.Execute("CREATE TABLE Users (Id INTEGER PRIMARY KEY, Name TEXT, Age INTEGER)");
        connection.Execute("INSERT INTO Users (Name, Age) VALUES ('Alice', 30), ('Bob', 25), ('Charlie', 35)");
        var users = connection.Query<User>("SELECT * FROM Users").ToList();
        var adults = users.Where(u => u.Age >= 30);
        foreach (var user in adults)
        {
            Console.WriteLine($"{user.Name} ({user.Age}歳)");
        }
    }
}
class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
}

Alice (30歳)
Charlie (35歳)

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 10);
        // 偶数だけ抽出
        var evens = numbers.Where(n => n % 2 == 0);
        foreach (var n in evens)
        {
            Console.WriteLine(n);
        }
    }
}

2
4
6
8
10

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var fruits = new[] { "apple", "banana", "cherry", "date" };
        // インデックスが偶数の要素だけ抽出
        var filtered = fruits.Where((fruit, index) => index % 2 == 0);
        foreach (var fruit in filtered)
        {
            Console.WriteLine(fruit);
        }
    }
}

apple
cherry

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var people = new[]
        {
            new { Name = "Alice", Age = 30, City = "Tokyo" },
            new { Name = "Bob", Age = 25, City = "Osaka" },
            new { Name = "Charlie", Age = 35, City = "Nagoya" }
        };
        var namesAndAges = people.Select(p => new { p.Name, p.Age });
        foreach (var item in namesAndAges)
        {
            Console.WriteLine($"{item.Name} ({item.Age}歳)");
        }
    }
}

Alice (30歳)
Bob (25歳)
Charlie (35歳)

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var students = new[]
        {
            new { Name = "Alice", Scores = new[] { 80, 90 } },
            new { Name = "Bob", Scores = new[] { 70, 85, 88 } },
            new { Name = "Charlie", Scores = new[] { 95 } }
        };
        // 全てのスコアを一つのシーケンスにまとめる
        var allScores = students.SelectMany(s => s.Scores);
        foreach (var score in allScores)
        {
            Console.WriteLine(score);
        }
    }
}

80
90
70
85
88
95

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 5, 2, 9, 1, 6 };
        var ascending = numbers.OrderBy(n => n);
        var descending = numbers.OrderByDescending(n => n);
        Console.WriteLine("昇順:");
        foreach (var n in ascending) Console.WriteLine(n);
        Console.WriteLine("降順:");
        foreach (var n in descending) Console.WriteLine(n);
    }
}

昇順:
1
2
5
6
9
降順:
9
6
5
2
1

using System;
using System.Linq;
class Program
{
    class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public string City { get; set; }
    }
    static void Main()
    {
        var people = new[]
        {
            new Person { Name = "Alice", Age = 30, City = "Tokyo" },
            new Person { Name = "Bob", Age = 25, City = "Osaka" },
            new Person { Name = "Charlie", Age = 30, City = "Osaka" },
            new Person { Name = "David", Age = 25, City = "Tokyo" }
        };
        var sorted = people
            .OrderBy(p => p.Age)
            .ThenBy(p => p.City);
        foreach (var p in sorted)
        {
            Console.WriteLine($"{p.Name}: {p.Age}歳, {p.City}");
        }
    }
}

Bob: 25歳, Osaka
David: 25歳, Tokyo
Charlie: 30歳, Osaka
Alice: 30歳, Tokyo

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var words = new[] { "apple", "banana", "apricot", "blueberry", "cherry" };
        var groups = words.GroupBy(w => w[0]); // 先頭文字でグループ化
        foreach (var group in groups)
        {
            Console.WriteLine($"Key: {group.Key}");
            foreach (var word in group)
            {
                Console.WriteLine($"  {word}");
            }
        }
    }
}

Key: a
  apple
  apricot
Key: b
  banana
  blueberry
Key: c
  cherry

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var people = new[]
        {
            new { Name = "Alice", City = "Tokyo", Age = 30 },
            new { Name = "Bob", City = "Osaka", Age = 25 },
            new { Name = "Charlie", City = "Tokyo", Age = 35 },
            new { Name = "David", City = "Osaka", Age = 40 }
        };
        var grouped = people.GroupBy(p => p.City)
            .Select(g => new
            {
                City = g.Key,
                Count = g.Count(),
                Oldest = g.OrderByDescending(p => p.Age).First()
            });
        foreach (var group in grouped)
        {
            Console.WriteLine($"{group.City}: {group.Count}人, 最年長は{group.Oldest.Name}({group.Oldest.Age}歳)");
        }
    }
}

Tokyo: 2人, 最年長はCharlie(35歳)
Osaka: 2人, 最年長はDavid(40歳)

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 1, 2, 2, 3, 4, 4, 5 };
        var distinct = numbers.Distinct();
        foreach (var n in distinct)
        {
            Console.WriteLine(n);
        }
    }
}

1
2
3
4
5

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var set1 = new[] { 1, 2, 3, 4 };
        var set2 = new[] { 3, 4, 5, 6 };
        var union = set1.Union(set2);
        var intersect = set1.Intersect(set2);
        var except = set1.Except(set2);
        Console.WriteLine("Union:");
        foreach (var n in union) Console.WriteLine(n);
        Console.WriteLine("Intersect:");
        foreach (var n in intersect) Console.WriteLine(n);
        Console.WriteLine("Except:");
        foreach (var n in except) Console.WriteLine(n);
    }
}

Union:
1
2
3
4
5
6
Intersect:
3
4
Except:
1
2

using System;
using System.Linq;
class Program
{
    class Employee
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }
    class Department
    {
        public int EmployeeId { get; set; }
        public string DeptName { get; set; }
    }
    static void Main()
    {
        var employees = new[]
        {
            new Employee { Id = 1, Name = "Alice" },
            new Employee { Id = 2, Name = "Bob" },
            new Employee { Id = 3, Name = "Charlie" }
        };
        var departments = new[]
        {
            new Department { EmployeeId = 1, DeptName = "HR" },
            new Department { EmployeeId = 2, DeptName = "IT" }
        };
        var query = employees.Join(
            departments,
            e => e.Id,
            d => d.EmployeeId,
            (e, d) => new { e.Name, d.DeptName }
        );
        foreach (var item in query)
        {
            Console.WriteLine($"{item.Name}: {item.DeptName}");
        }
    }
}

Alice: HR
Bob: IT

using System;
using System.Linq;
class Program
{
    class Employee
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }
    class Department
    {
        public int EmployeeId { get; set; }
        public string DeptName { get; set; }
    }
    static void Main()
    {
        var employees = new[]
        {
            new Employee { Id = 1, Name = "Alice" },
            new Employee { Id = 2, Name = "Bob" },
            new Employee { Id = 3, Name = "Charlie" }
        };
        var departments = new[]
        {
            new Department { EmployeeId = 1, DeptName = "HR" },
            new Department { EmployeeId = 2, DeptName = "IT" }
        };
        var query = employees.GroupJoin(
            departments,
            e => e.Id,
            d => d.EmployeeId,
            (e, ds) => new { e.Name, Depts = ds.DefaultIfEmpty() }
        );
        foreach (var item in query)
        {
            foreach (var dept in item.Depts)
            {
                var deptName = dept?.DeptName ?? "なし";
                Console.WriteLine($"{item.Name}: {deptName}");
            }
        }
    }
}

Alice: HR
Bob: IT
Charlie: なし

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var colors = new[] { "Red", "Green" };
        var shapes = new[] { "Circle", "Square" };
        var crossJoin = colors.SelectMany(
            color => shapes,
            (color, shape) => $"{color} {shape}"
        );
        foreach (var item in crossJoin)
        {
            Console.WriteLine(item);
        }
    }
}

Red Circle
Red Square
Green Circle
Green Square

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 100);
        Console.WriteLine($"Count: {numbers.Count()}");
        Console.WriteLine($"LongCount: {numbers.LongCount()}");
    }
}

Count: 100
LongCount: 100

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 1, 2, 3, 4, 5 };
        Console.WriteLine($"Sum: {numbers.Sum()}");
        Console.WriteLine($"Average: {numbers.Average()}");
    }
}

Sum: 15
Average: 3

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 10, 20, 5, 30, 15 };
        Console.WriteLine($"Min: {numbers.Min()}");
        Console.WriteLine($"Max: {numbers.Max()}");
    }
}

Min: 5
Max: 30

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var words = new[] { "LINQ", "is", "powerful" };
        var sentence = words.Aggregate((acc, word) => acc + " " + word);
        Console.WriteLine(sentence);
    }
}

LINQ is powerful

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 10);
        var firstThree = numbers.Take(3);
        var skipThree = numbers.Skip(3);
        Console.WriteLine("Take(3):");
        foreach (var n in firstThree)
        {
            Console.WriteLine(n);
        }
        Console.WriteLine("Skip(3):");
        foreach (var n in skipThree)
        {
            Console.WriteLine(n);
        }
    }
}

Take(3):
1
2
3
Skip(3):
4
5
6
7
8
9
10

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var items = Enumerable.Range(1, 50);
        int pageSize = 10;
        int pageNumber = 3; // 3ページ目(0ベースではなく1ベース)
        var pageItems = items
            .Skip((pageNumber - 1) * pageSize)
            .Take(pageSize);
        Console.WriteLine($"Page {pageNumber}:");
        foreach (var item in pageItems)
        {
            Console.WriteLine(item);
        }
    }
}

Page 3:
21
22
23
24
25
26
27
28
29
30

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 2, 4, 6, 7, 8, 10 };
        var takeWhileEven = numbers.TakeWhile(n => n % 2 == 0);
        var skipWhileEven = numbers.SkipWhile(n => n % 2 == 0);
        Console.WriteLine("TakeWhile (偶数の間):");
        foreach (var n in takeWhileEven)
        {
            Console.WriteLine(n);
        }
        Console.WriteLine("SkipWhile (偶数の間スキップ):");
        foreach (var n in skipWhileEven)
        {
            Console.WriteLine(n);
        }
    }
}

TakeWhile (偶数の間):
2
4
6
SkipWhile (偶数の間スキップ):
7
8
10

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 1, 3, 5, 7, 9 };
        bool anyEven = numbers.Any(n => n % 2 == 0);
        bool allOdd = numbers.All(n => n % 2 != 0);
        bool containsFive = numbers.Contains(5);
        Console.WriteLine($"Any even? {anyEven}");
        Console.WriteLine($"All odd? {allOdd}");
        Console.WriteLine($"Contains 5? {containsFive}");
    }
}

Any even? False
All odd? True
Contains 5? True

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 2, 4, 6, 8 };
        int firstEven = numbers.First(n => n % 2 == 0);
        int firstGreaterThanTen = numbers.FirstOrDefault(n => n > 10);
        Console.WriteLine($"First even: {firstEven}");
        Console.WriteLine($"First > 10 or default: {firstGreaterThanTen}");
    }
}

First even: 2
First > 10 or default: 0

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = new[] { 5 };
        int single = numbers.Single();
        int singleOrDefault = numbers.SingleOrDefault(n => n > 10);
        Console.WriteLine($"Single: {single}");
        Console.WriteLine($"SingleOrDefault (条件に合わず): {singleOrDefault}");
    }
}

Single: 5
SingleOrDefault (条件に合わず): 0

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var letters = new[] { "a", "b", "c" };
        string second = letters.ElementAt(1);
        string fifthOrDefault = letters.ElementAtOrDefault(4);
        Console.WriteLine($"ElementAt(1): {second}");
        Console.WriteLine($"ElementAtOrDefault(4): {(fifthOrDefault ?? "null")}");
    }
}

ElementAt(1): b
ElementAtOrDefault(4): null

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 10);
        foreach (var n in numbers)
        {
            Console.WriteLine(n);
        }
    }
}

1
2
3
4
5
6
7
8
9
10

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var repeated = Enumerable.Repeat("Hello", 5);
        foreach (var s in repeated)
        {
            Console.WriteLine(s);
        }
    }
}

Hello
Hello
Hello
Hello
Hello

using System;
using System.Linq;
class Program
{
    static void Main()
    {
        var empty = Enumerable.Empty<int>();
        Console.WriteLine($"Count: {empty.Count()}");
    }
}

Count: 0

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        foreach (var even in GenerateEvenNumbers(10))
        {
            Console.WriteLine(even);
        }
    }
    static IEnumerable<int> GenerateEvenNumbers(int max)
    {
        for (int i = 1; i <= max; i++)
        {
            if (i % 2 == 0)
            {
                yield return i; // 遅延生成
            }
        }
    }
}

2
4
6
8
10

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        foreach (var n in NaturalNumbers())
        {
            if (n > 10) break; // 無限ループ防止
            Console.WriteLine(n);
        }
    }
    static IEnumerable<int> NaturalNumbers()
    {
        int i = 1;
        while (true)
        {
            yield return i++;
        }
    }
}

1
2
3
4
5
6
7
8
9
10

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 5);
        // ToListでList<int>に変換
        List<int> list = numbers.ToList();
        // ToArrayでint[]に変換
        int[] array = numbers.ToArray();
        Console.WriteLine("Listの要素:");
        foreach (var n in list)
        {
            Console.WriteLine(n);
        }
        Console.WriteLine("配列の要素:");
        foreach (var n in array)
        {
            Console.WriteLine(n);
        }
    }
}

Listの要素:
1
2
3
4
5
配列の要素:
1
2
3
4
5

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    class Person
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }
    static void Main()
    {
        var people = new[]
        {
            new Person { Id = 1, Name = "Alice" },
            new Person { Id = 2, Name = "Bob" },
            new Person { Id = 3, Name = "Charlie" }
        };
        Dictionary<int, string> dict = people.ToDictionary(p => p.Id, p => p.Name);
        foreach (var kv in dict)
        {
            Console.WriteLine($"Key: {kv.Key}, Value: {kv.Value}");
        }
    }
}

Key: 1, Value: Alice
Key: 2, Value: Bob
Key: 3, Value: Charlie

using System;
using System.Linq;
class Program
{
    class Fruit
    {
        public string Name { get; set; }
        public string Color { get; set; }
    }
    static void Main()
    {
        var fruits = new[]
        {
            new Fruit { Name = "Apple", Color = "Red" },
            new Fruit { Name = "Strawberry", Color = "Red" },
            new Fruit { Name = "Banana", Color = "Yellow" },
            new Fruit { Name = "Lemon", Color = "Yellow" }
        };
        var lookup = fruits.ToLookup(f => f.Color);
        foreach (var group in lookup)
        {
            Console.WriteLine($"Color: {group.Key}");
            foreach (var fruit in group)
            {
                Console.WriteLine($"  {fruit.Name}");
            }
        }
    }
}

Color: Red
  Apple
  Strawberry
Color: Yellow
  Banana
  Lemon

using System;
using System.Collections;
using System.Linq;
class Program
{
    static void Main()
    {
        ArrayList list = new ArrayList { 1, 2, 3, 4 };
        var casted = list.Cast<int>();
        foreach (var n in casted)
        {
            Console.WriteLine(n);
        }
    }
}

1
2
3
4

using System;
using System.Collections;
using System.Linq;
class Program
{
    static void Main()
    {
        ArrayList list = new ArrayList { 1, "two", 3, "four", 5 };
        var ints = list.OfType<int>();
        foreach (var n in ints)
        {
            Console.WriteLine(n);
        }
    }
}

1
3
5

using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 5);
        // 遅延実行のままだとforeachごとに処理が走る
        Console.WriteLine("遅延実行:");
        foreach (var n in numbers.Where(x => x % 2 == 0))
            Console.WriteLine(n);
        foreach (var n in numbers.Where(x => x % 2 == 0))
            Console.WriteLine(n);
        // ToListで即時実行し結果をキャッシュ
        var cached = numbers.Where(x => x % 2 == 0).ToList();
        Console.WriteLine("ToListでキャッシュ:");
        foreach (var n in cached)
            Console.WriteLine(n);
        foreach (var n in cached)
            Console.WriteLine(n);
    }
}

遅延実行:
2
4
2
4
ToListでキャッシュ:
2
4
2
4

using System;
using System.Linq;
class Program
{
    static int counter = 0;
    static void Main()
    {
        var numbers = Enumerable.Range(1, 3).Select(n =>
        {
            counter++;
            return n;
        });
        // 2回列挙するとSelectの処理が2回走る
        foreach (var n in numbers)
            Console.WriteLine(n);
        foreach (var n in numbers)
            Console.WriteLine(n);
        Console.WriteLine($"処理回数: {counter}");
    }
}

1
2
3
1
2
3
処理回数: 6

using System;
using System.Linq;
using System.Threading;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 20);
        var parallelQuery = numbers.AsParallel()
            .Select(n =>
            {
                Console.WriteLine($"Processing {n} on thread {Thread.CurrentThread.ManagedThreadId}");
                return n * n;
            });
        foreach (var result in parallelQuery)
        {
            Console.WriteLine(result);
        }
    }
}

Processing 1 on thread 4
Processing 2 on thread 5
Processing 3 on thread 6
...
1
4
9
...

using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Threading.Tasks;
class Program
{
    static void Main()
    {
        var numbers = Enumerable.Range(1, 1000);
        var results = new ConcurrentBag<int>();
        numbers.AsParallel().ForAll(n =>
        {
            // ConcurrentBagはスレッドセーフ
            results.Add(n * n);
        });
        Console.WriteLine($"結果数: {results.Count}");
    }
}

結果数: 1000

using System;
using System.Collections.Generic;
public static class EnumerableExtensions
{
    public static IEnumerable<T> Memoize<T>(this IEnumerable<T> source)
    {
        var cache = new List<T>();
        bool fullyEnumerated = false;
        foreach (var item in source)
        {
            cache.Add(item);
            yield return item;
        }
        fullyEnumerated = true;
        int index = 0;
        while (true)
        {
            if (index < cache.Count)
            {
                yield return cache[index++];
            }
            else if (fullyEnumerated)
            {
                yield break;
            }
        }
    }
}
class Program
{
    static void Main()
    {
        var numbers = GenerateNumbers();
        var memoized = numbers.Memoize();
        foreach (var n in memoized)
            Console.WriteLine(n);
        Console.WriteLine("再列挙:");
        foreach (var n in memoized)
            Console.WriteLine(n);
    }
    static IEnumerable<int> GenerateNumbers()
    {
        Console.WriteLine("生成開始");
        for (int i = 1; i <= 3; i++)
        {
            Console.WriteLine($"生成中: {i}");
            yield return i;
        }
    }
}

生成開始
生成中: 1
1
生成中: 2
2
生成中: 3
3
再列挙:
1
2
3

using System;
using System.Linq;
class Program
{
    class Person
    {
        public string Name { get; set; }
        public string City { get; set; }
    }
    static void Main()
    {
        var people = new[]
        {
            new Person { Name = "Alice", City = "Tokyo" },
            new Person { Name = "Bob", City = null },
            new Person { Name = "Charlie", City = "Osaka" },
            null
        };
        // nullチェックを含めた安全なクエリ
        var filtered = people
            .Where(p => p != null && !string.IsNullOrEmpty(p.City))
            .Select(p => p.Name + " lives in " + p.City);
        foreach (var s in filtered)
        {
            Console.WriteLine(s);
        }
    }
}