C# 12 - Quick Reference

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: csharp for comprehensive documentation.

Records

// Positional record (immutable) public record UserDto(int Id, string Name, string Email);

// Record with additional members public record OrderDto(int Id, decimal Total) { public string FormattedTotal => Total.ToString("C"); }

// Record struct (value type, C# 10) public readonly record struct Point(double X, double Y);

// With-expression (non-destructive mutation) var updated = user with { Name = "New Name" };

Pattern Matching (C# 12)

// Switch expression string GetDiscount(Customer customer) => customer switch { { Type: CustomerType.Premium, YearsActive: > 5 } => "30%", { Type: CustomerType.Premium } => "20%", { Type: CustomerType.Regular, YearsActive: > 3 } => "10%", _ => "0%", };

// List patterns (C# 11) var result = numbers switch { [1, 2, 3] => "exact match", [1, .., 3] => "starts with 1, ends with 3", [_, > 5, ..] => "second element > 5", [] => "empty", _ => "other", };

// Type pattern with when if (shape is Circle { Radius: > 10 } circle) { Console.WriteLine($"Large circle: {circle.Radius}"); }

Nullable Reference Types

// Enable in .csproj: <Nullable>enable</Nullable>

public class UserService { // Nullable return type public User? FindById(int id) => _users.FirstOrDefault(u => u.Id == id);

// Non-nullable parameter
public void Update(User user)
{
    ArgumentNullException.ThrowIfNull(user);
    // user is guaranteed non-null here
}

// Null-conditional and coalescing
public string GetDisplayName(User? user)
    => user?.Name ?? "Unknown";

// Required members (C# 11)
public required string Name { get; init; }

}

Async/Await Patterns

// Basic async method public async Task<User> GetUserAsync(int id) { var user = await _repository.FindAsync(id); return user ?? throw new NotFoundException($"User {id} not found"); }

// Parallel async public async Task<(User[], Order[])> GetDashboardDataAsync(int userId) { var usersTask = _userService.GetAllAsync(); var ordersTask = _orderService.GetByUserAsync(userId); await Task.WhenAll(usersTask, ordersTask); return (usersTask.Result, ordersTask.Result); }

// IAsyncEnumerable public async IAsyncEnumerable<User> GetUsersStreamAsync( [EnumeratorCancellation] CancellationToken ct = default) { await foreach (var user in _context.Users.AsAsyncEnumerable().WithCancellation(ct)) { yield return user; } }

// ValueTask for hot paths public ValueTask<User?> GetCachedUserAsync(int id) { if (_cache.TryGetValue(id, out var user)) return ValueTask.FromResult<User?>(user); return new ValueTask<User?>(LoadUserAsync(id)); }

LINQ

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

// GroupBy var grouped = orders .GroupBy(o => o.Status) .Select(g => new { Status = g.Key, Count = g.Count(), Total = g.Sum(o => o.Amount) });

// Aggregate var summary = orders.Aggregate( new { Count = 0, Total = 0m }, (acc, order) => new { Count = acc.Count + 1, Total = acc.Total + order.Amount });

// Chunk (C# 12 / .NET 8) var batches = users.Chunk(100); // IEnumerable<User[]>

Primary Constructors (C# 12)

// Class primary constructor public class UserService(IUserRepository repository, ILogger<UserService> logger) { public async Task<User> GetByIdAsync(int id) { logger.LogInformation("Getting user {Id}", id); return await repository.GetByIdAsync(id) ?? throw new NotFoundException($"User {id} not found"); } }

// Record primary constructor (already existed) public record UserDto(int Id, string Name, string Email);

Collection Expressions (C# 12)

// Array int[] numbers = [1, 2, 3, 4, 5];

// List List<string> names = ["Alice", "Bob", "Charlie"];

// Spread int[] first = [1, 2, 3]; int[] second = [4, 5, 6]; int[] combined = [..first, ..second]; // [1, 2, 3, 4, 5, 6]

Anti-Patterns

Anti-Pattern Why It's Bad Correct Approach

.Result / .Wait()

Deadlocks Use async/await

async void

Exceptions lost Use async Task

Mutable DTOs Unexpected mutations Use record types

No null checking NullReferenceException Enable nullable references

String concatenation in loops Memory pressure Use StringBuilder

Quick Troubleshooting

Issue Likely Cause Solution

Nullable warning Missing null check Add ? , ?? , or null guard

Deadlock .Result in async context Use await

LINQ multiple enumeration Iterating IEnumerable twice Call .ToList() first

Record equality fails Reference type property Override Equals or use value types