Developing Csharp

C# Development

Apply these patterns when working with C# code. When both developing-csharp and testing-xunit apply, testing-xunit takes precedence in test files.

Naming Conventions

Project Structure

src/
├── MyApp.Api/Controllers/
├── MyApp.Core/Entities/, Interfaces/, Services/
├── MyApp.Infrastructure/Repositories/, Data/
tests/
├── MyApp.UnitTests/
├── MyApp.IntegrationTests/

Dependency Injection

Register services in Program.cs:

builder.Services.AddScoped<IUserRepository, UserRepository>();
builder.Services.AddSingleton<ICacheService, RedisCacheService>();
builder.Services.AddTransient<IEmailSender, SmtpEmailSender>();

Inject via constructor:

public class UserService : IUserService
{
    private readonly IUserRepository _userRepository;
    private readonly ILogger<UserService> _logger;

    public UserService(IUserRepository userRepository, ILogger<UserService> logger)
    {
        _userRepository = userRepository;
        _logger = logger;
    }
}

Async/Await

Always use Async suffix. Always await or return the task. Use ConfigureAwait(false) in libraries. Support CancellationToken.

public async Task<User> GetUserAsync(int id, CancellationToken ct = default)
{
    return await _context.Users.FirstOrDefaultAsync(u => u.Id == id, ct);
}

Do not use async void except for event handlers. Do not block with .Result or .Wait(). Do not ignore tasks with _ = ProcessAsync().

LINQ

Prefer method syntax:

var activeUsers = users
    .Where(u => u.IsActive)
    .OrderBy(u => u.LastName)
    .Select(u => new UserDto(u.Id, u.FullName))
    .ToList();

Materialize once to avoid multiple enumeration:

var activeUsers = users.Where(u => u.IsActive).ToList();
var count = activeUsers.Count;
var first = activeUsers.First();

Error Handling

Use custom exception hierarchy:

public class DomainException : Exception { }
public class NotFoundException : DomainException { }
public class ValidationException : DomainException
{
    public IReadOnlyList<string> Errors { get; }
}

Result pattern alternative:

public class Result<T>
{
    public bool IsSuccess { get; }
    public T? Value { get; }
    public string? Error { get; }

    public static Result<T> Success(T value) => new(value);
    public static Result<T> Failure(string error) => new(error);
}

Testing with xUnit

public class UserServiceTests
{
    private readonly Mock<IUserRepository> _mockRepo;
    private readonly UserService _sut;

    public UserServiceTests()
    {
        _mockRepo = new Mock<IUserRepository>();
        _sut = new UserService(_mockRepo.Object);
    }

    [Fact]
    public async Task GetByIdAsync_WithValidId_ReturnsUser()
    {
        var expectedUser = new User { Id = 1, Name = "John" };
        _mockRepo.Setup(r => r.GetByIdAsync(1)).ReturnsAsync(expectedUser);

        var result = await _sut.GetByIdAsync(1);

        Assert.NotNull(result);
        Assert.Equal("John", result.Name);
    }
}

Entity Framework

public class AppDbContext : DbContext
{
    public DbSet<User> Users => Set<User>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);
    }
}

Avoid the repository pattern with Entity Framework.

Requirements

1. Use var for obvious types
2. Prefer readonly fields
3. Use record for DTOs: public record UserDto(int Id, string Name);
4. Use nullable reference types and null-coalescing operators
5. Use using statements for disposable resources
6. Use "" not string.Empty
7. Use collection expressions (C# 12+): int[] numbers = [1, 2, 3];