LINQ

using System;
using System.Collections.Generic;
using System.Linq;

record User(string Name, int Age, string City, decimal Salary);

var users = new[] {
    new User("Ada", 32, "Sydney",    120_000m),
    new User("Bo",  28, "Sydney",     95_000m),
    new User("Cy",  41, "Melbourne", 140_000m),
    new User("Di",  22, "Brisbane",   65_000m),
};

// 1) Filter + project
var names = users
    .Where(u => u.Age >= 30)
    .OrderByDescending(u => u.Salary)
    .Select(u => u.Name)
    .ToList();

// 2) Aggregations
decimal totalPay = users.Sum(u => u.Salary);
double  avgAge   = users.Average(u => u.Age);
int     headcount = users.Count();
int     count30  = users.Count(u => u.Age >= 30);

// 3) Group
var byCity = users.GroupBy(u => u.City)
    .Select(g => new { City = g.Key, Total = g.Sum(u => u.Salary), Count = g.Count() })
    .ToList();

foreach (var row in byCity)
    Console.WriteLine($"{row.City}: {row.Count} users, payroll {row.Total:C}");

// 4) Join
var orders = new[] { new { UserName = "Ada", Total = 9.99m }, new { UserName = "Ada", Total = 49.99m } };
var joined = users.Join(
    orders,
    u => u.Name,
    o => o.UserName,
    (u, o) => new { u.Name, o.Total });

// 5) Distinct, Take, Skip
var uniqueCities = users.Select(u => u.City).Distinct();
var topThree     = users.OrderByDescending(u => u.Salary).Take(3);
var pageTwo      = users.OrderBy(u => u.Name).Skip(20).Take(20);

// 6) First / Single / Any / All
var first = users.First(u => u.Salary > 100_000);            // throws if none
var maybe = users.FirstOrDefault(u => u.Salary > 1_000_000); // null if none
bool hasMin = users.All(u => u.Age >= 18);
bool anyOld = users.Any(u => u.Age >= 65);

// 7) Anonymous projection vs records
var shaped = users.Select(u => new { u.Name, u.City });   // anonymous
// Better in libraries: record DisplayUser(string Name, string City);

// 8) Query syntax — same result, different shape
var q = from u in users
        where u.Age >= 30
        orderby u.Salary descending
        select u.Name;

// 9) LINQ to Entity Framework — translates to SQL
var topByCity = await db.Users
    .Where(u => u.Active)
    .GroupBy(u => u.City)
    .Select(g => new { g.Key, Total = g.Sum(u => u.Salary) })
    .OrderByDescending(g => g.Total)
    .Take(10)
    .ToListAsync();

// 10) Be careful of double-enumeration
var q2 = users.Where(u => u.Age >= 30);
var list = q2.ToList();    // materialise ONCE
foreach (var u in list) Console.WriteLine(u.Name);
int c = list.Count;

// 11) Chunk (5+), Zip, Distinct(comparer)
var batches = Enumerable.Range(1, 23).Chunk(5);
var paired  = users.Zip(orders, (u, o) => new { u.Name, o.Total });

int[] nums = {1, 2, 3, 4, 5};
var evens = nums.Where(n => n % 2 == 0).Sum();   // 6
var names = users.OrderBy(u => u.Name).Select(u => u.Name);