LINQ: запросы к коллекциям в C#

1using System.Linq;
2
3var numbers = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
4
5// Синтаксис запросов
6var evenNumbers = from n in numbers
7                  where n % 2 == 0
8                  select n;
9
10// Синтаксис методов (то же самое)
11var evenNumbers2 = numbers.Where(n => n % 2 == 0);

1var adults = people.Where(p => p.Age >= 18);
2
3// С индексом
4var indexed = people.Where((p, index) => p.Age >= 18 && index < 5);

1var names = people.Select(p => p.Name);
2
3// Анонимные типы
4var projections = people.Select(p => new
5{
6    p.Name,
7    p.Age,
8    IsAdult = p.Age >= 18
9});

1var sorted = people.OrderBy(p => p.Age);
2var sortedDesc = people.OrderByDescending(p => p.Age);
3
4// Многократная сортировка
5var multiSorted = people
6    .OrderBy(p => p.LastName)
7    .ThenBy(p => p.FirstName);

1// GroupBy
2var grouped = people.GroupBy(p => p.City);
3
4foreach (var group in grouped)
5{
6    Console.WriteLine($"City: {group.Key}");
7    foreach (var person in group)
8    {
9        Console.WriteLine($"  {person.Name}");
10    }
11}
12
13// Агрегация в группах
14var cityStats = people
15    .GroupBy(p => p.City)
16    .Select(g => new
17    {
18        City = g.Key,
19        Count = g.Count(),
20        AverageAge = g.Average(p => p.Age),
21        MaxAge = g.Max(p => p.Age)
22    });

1var result = from p in people
2             join d in departments on p.DepartmentId equals d.Id
3             select new { p.Name, Department = d.Name };

1var result = departments.GroupJoin(
2    people,
3    d => d.Id,
4    p => p.DepartmentId,
5    (d, employees) => new
6    {
7        Department = d.Name,
8        Employees = employees.ToList()
9    }
10);

1var numbers = new[] { 1, 2, 3, 4, 5 };
2
3int sum = numbers.Sum();           // 15
4int min = numbers.Min();           // 1
5int max = numbers.Max();           // 5
6double avg = numbers.Average();    // 3.0
7int count = numbers.Count();       // 5
8
9// Кастомная агрегация
10int product = numbers.Aggregate((a, b) => a * b); // 120

1// ToList / ToArray / ToDictionary
2List<int> list = numbers.ToList();
3int[] array = numbers.ToArray();
4var dict = people.ToDictionary(p => p.Id);
5
6// Cast
7var ints = new object[] { 1, 2, 3 };
8var casted = ints.Cast<int>();
9
10// OfType
11var mixed = new object[] { 1, "two", 3.0 };
12var doubles = mixed.OfType<double>(); // [3.0]

1var first = numbers.First();              // 1
2var firstEven = numbers.First(n => n % 2 == 0); // 2
3var firstOrDefault = numbers.FirstOrDefault(n => n > 100); // default
4
5var last = numbers.Last();                // 10
6var single = numbers.Single(n => n == 5); // 5
7
8var elementAt = numbers.ElementAt(3);     // 4

1var first5 = numbers.Take(5);        // [1, 2, 3, 4, 5]
2var skip5 = numbers.Skip(5);         // [6, 7, 8, 9, 10]
3var takeWhile = numbers.TakeWhile(n => n < 5); // [1, 2, 3, 4]
4var skipWhile = numbers.SkipWhile(n => n < 5); // [5, 6, 7, 8, 9, 10]

1var unique = new[] { 1, 1, 2, 3, 3 }.Distinct(); // [1, 2, 3]
2
3var set1 = new[] { 1, 2, 3 };
4var set2 = new[] { 2, 3, 4 };
5var union = set1.Union(set2);       // [1, 2, 3, 4]
6var intersect = set1.Intersect(set2); // [2, 3]
7var except = set1.Except(set2);     // [1]