public record User(string Id, string Name, string Email);

public class UserService
{
    public async Task<User?> GetUserAsync(string id)
    {
        return await _repository.FindByIdAsync(id);
    }
}

// Records for immutable data
public record User(string Id, string Name, string Email)
{
    public string DisplayName => Name.ToUpperInvariant();
}

// With-expressions for copies
var updated = user with { Name = "New Name" };

// Nullable reference types
public User? FindUser(string id)
{
    return users.FirstOrDefault(u => u.Id == id);
}

public async Task<List<User>> GetUsersAsync()
{
    // Parallel async operations
    var tasks = ids.Select(id => GetUserAsync(id));
    var users = await Task.WhenAll(tasks);
    return users.ToList();
}

// Async streams
public async IAsyncEnumerable<User> StreamUsersAsync()
{
    await foreach (var user in _repository.GetAllAsync())
    {
        yield return user;
    }
}

// Cancellation
public async Task<User> GetUserAsync(string id, CancellationToken ct)
{
    return await _client.GetFromJsonAsync<User>($"/users/{id}", ct);
}

// Query syntax
var adults = from user in users
             where user.Age >= 18
             orderby user.Name
             select user;

// Method syntax (preferred)
var result = users
    .Where(u => u.IsActive)
    .OrderBy(u => u.Name)
    .Select(u => new UserDto(u.Id, u.Name))
    .ToList();

// Grouping
var byCountry = users
    .GroupBy(u => u.Country)
    .ToDictionary(g => g.Key, g => g.ToList());

string GetStatus(object obj) => obj switch
{
    User { IsActive: true } => "Active user",
    User { IsActive: false } => "Inactive user",
    null => "No data",
    _ => "Unknown"
};

// List patterns
if (numbers is [var first, _, var last])
{
    Console.WriteLine($"First: {first}, Last: {last}");
}