dotnet-backend-patterns skill

.NET Backend Development Patterns

Master C#/.NET patterns for building production-grade APIs, MCP servers, and enterprise backends with modern best practices (2024/2025).

When to Use This Skill

• Developing new .NET Web APIs or MCP servers
• Reviewing C# code for quality and performance
• Designing service architectures with dependency injection
• Implementing caching strategies with Redis
• Writing unit and integration tests
• Optimizing database access with EF Core or Dapper
• Configuring applications with IOptions pattern
• Handling errors and implementing resilience patterns

Core Concepts

1. Project Structure (Clean Architecture)

src/
├── Domain/                     # Core business logic (no dependencies)
│   ├── Entities/
│   ├── Interfaces/
│   ├── Exceptions/
│   └── ValueObjects/
├── Application/                # Use cases, DTOs, validation
│   ├── Services/
│   ├── DTOs/
│   ├── Validators/
│   └── Interfaces/
├── Infrastructure/             # External implementations
│   ├── Data/                   # EF Core, Dapper repositories
│   ├── Caching/                # Redis, Memory cache
│   ├── External/               # HTTP clients, third-party APIs
│   └── DependencyInjection/    # Service registration
└── Api/                        # Entry point
    ├── Controllers/            # Or MinimalAPI endpoints
    ├── Middleware/
    ├── Filters/
    └── Program.cs

2. Dependency Injection Patterns

// Service registration by lifetime
public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddApplicationServices(
        this IServiceCollection services,
        IConfiguration configuration)
    {
        // Scoped: One instance per HTTP request
        services.AddScoped<IProductService, ProductService>();
        services.AddScoped<IOrderService, OrderService>();

        // Singleton: One instance for app lifetime
        services.AddSingleton<ICacheService, RedisCacheService>();
        services.AddSingleton<IConnectionMultiplexer>(_ =>
            ConnectionMultiplexer.Connect(configuration["Redis:Connection"]!));

        // Transient: New instance every time
        services.AddTransient<IValidator<CreateOrderRequest>, CreateOrderValidator>();

        // Options pattern for configuration
        services.Configure<CatalogOptions>(configuration.GetSection("Catalog"));
        services.Configure<RedisOptions>(configuration.GetSection("Redis"));

        // Factory pattern for conditional creation
        services.AddScoped<IPriceCalculator>(sp =>
        {
            var options = sp.GetRequiredService<IOptions<PricingOptions>>().Value;
  

3. Async/Await Patterns

// ✅ CORRECT: Async all the way down
public async Task<Product> GetProductAsync(string id, CancellationToken ct = default)
{
    return await _repository.GetByIdAsync(id, ct);
}

// ✅ CORRECT: Parallel execution with WhenAll
public async Task<(Stock, Price)> GetStockAndPriceAsync(
    string productId,
    CancellationToken ct = default)
{
    var stockTask = _stockService.GetAsync(productId, ct);
    var priceTask = _priceService.GetAsync(productId, ct);

    await Task.WhenAll(stockTask, priceTask);

    return (await stockTask, await priceTask);
}

// ✅ CORRECT: ConfigureAwait in libraries
public async Task<T> LibraryMethodAsync<T>(CancellationToken ct = default)
{
    var result = await _httpClient.GetAsync(url, ct).ConfigureAwait(false);
    return await result.Content.ReadFromJsonAsync<T>(ct).ConfigureAwait(false);
}

// ✅ CORRECT: ValueTask for hot paths with caching
public ValueTask<Product?> GetCachedProductAsync(string id)
{
    if (_cache.TryGetValue(id, out Product? product))
        return ValueTask.FromResult(product);

    return new ValueTask<Product?>(GetFromDatabaseAsync(id));
}

// ❌ WRONG: Blocking on async (deadlock risk)
var result = GetProductAsync(id).Result

4. Configuration with IOptions

// Configuration classes
public class CatalogOptions
{
    public const string SectionName = "Catalog";

    public int DefaultPageSize { get; set; } = 50;
    public int MaxPageSize { get; set; } = 200;
    public TimeSpan CacheDuration { get; set; } = TimeSpan.FromMinutes(15);
    public bool EnableEnrichment { get; set; } = true;
}

public class RedisOptions
{
    public const string SectionName = "Redis";

    public string Connection { get; set; } = "localhost:6379";
    public string KeyPrefix { get; set; } = "mcp:";
    public int Database { get; set; } = 0;
}

// appsettings.json
{
    "Catalog": {
        "DefaultPageSize": 50,
        "MaxPageSize": 200,
        "CacheDuration": "00:15:00",
        "EnableEnrichment": true
    },
    "Redis": {
        "Connection": "localhost:6379",
        "KeyPrefix": "mcp:",
        "Database": 0
    }
}

// Registration
services.Configure<CatalogOptions>(configuration.GetSection(CatalogOptions.SectionName));
services.Configure<RedisOptions>(configuration.GetSection(RedisOptions.SectionName));

// Usage with IOptions (singleton, read once at startup)
public class CatalogService
{
    private readonly CatalogOptions _options;

    publ

5. Result Pattern (Avoiding Exceptions for Flow Control)

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

    private Result(bool isSuccess, T? value, string? error, string? errorCode)
    {
        IsSuccess = isSuccess;
        Value = value;
        Error = error;
        ErrorCode = errorCode;
    }

    public static Result<T> Success(T value) => new(true, value, null, null);
    public static Result<T> Failure(string error, string? code = null) => new(false, default, error, code);

    public Result<TNew> Map<TNew>(Func<T, TNew> mapper) =>
        IsSuccess ? Result<TNew>.Success(mapper(Value!)) : Result<TNew>.Failure(Error!, ErrorCode);

    public async Task<Result<TNew>> MapAsync<TNew>(Func<T, Task<TNew>> mapper) =>
        IsSuccess ? Result<TNew>.Success(await mapper(Value!)) : Result<TNew>.Failure(Error!, ErrorCode);
}

// Usage in service
public async Task<Result<Order>> CreateOrderAsync(CreateOrderRequest request, CancellationToken ct)
{
    // Validation
    var validation = await _validator.ValidateAsync(request, ct);
    if (!validation.IsValid)
        return Result<Order>.Failure(
    

Data Access Patterns

Entity Framework Core

// DbContext configuration
public class AppDbContext : DbContext
{
    public DbSet<Product> Products => Set<Product>();
    public DbSet<Order> Orders => Set<Order>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Apply all configurations from assembly
        modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);

        // Global query filters
        modelBuilder.Entity<Product>().HasQueryFilter(p => !p.IsDeleted);
    }
}

// Entity configuration
public class ProductConfiguration : IEntityTypeConfiguration<Product>
{
    public void Configure(EntityTypeBuilder<Product> builder)
    {
        builder.ToTable("Products");

        builder.HasKey(p => p.Id);
        builder.Property(p => p.Id).HasMaxLength(40);
        builder.Property(p => p.Name).HasMaxLength(200).IsRequired();
        builder.Property(p => p.Price).HasPrecision(18, 2);

        builder.HasIndex(p => p.Sku).IsUnique();
        builder.HasIndex(p => new { p.CategoryId, p.Name });

        builder.HasMany(p => p.OrderItems)
            .WithOne(oi => oi.Product)
            .HasForeignKey(oi => oi.ProductId);
    }
}

// Repository with EF Core
pub

Dapper for Performance

public class DapperProductRepository : IProductRepository
{
    private readonly IDbConnection _connection;

    public async Task<Product?> GetByIdAsync(string id, CancellationToken ct = default)
    {
        const string sql = """
            SELECT Id, Name, Sku, Price, CategoryId, Stock, CreatedAt
            FROM Products
            WHERE Id = @Id AND IsDeleted = 0
            """;

        return await _connection.QueryFirstOrDefaultAsync<Product>(
            new CommandDefinition(sql, new { Id = id }, cancellationToken: ct));
    }

    public async Task<IReadOnlyList<Product>> SearchAsync(
        ProductSearchCriteria criteria,
        CancellationToken ct = default)
    {
        var sql = new StringBuilder("""
            SELECT Id, Name, Sku, Price, CategoryId, Stock, CreatedAt
            FROM Products
            WHERE IsDeleted = 0
            """);

        var parameters = new DynamicParameters();

        if (!string.IsNullOrWhiteSpace(criteria.SearchTerm))
        {
            sql.Append(" AND Name LIKE @SearchTerm");
            parameters.Add("SearchTerm", $"%{criteria.SearchTerm}%");
        }

        if (criteria.CategoryId.HasValue)
        {
          

Caching Patterns

Multi-Level Cache with Redis

public class CachedProductService : IProductService
{
    private readonly IProductRepository _repository;
    private readonly IMemoryCache _memoryCache;
    private readonly IDistributedCache _distributedCache;
    private readonly ILogger<CachedProductService> _logger;

    private static readonly TimeSpan MemoryCacheDuration = TimeSpan.FromMinutes(1);
    private static readonly TimeSpan DistributedCacheDuration = TimeSpan.FromMinutes(15);

    public async Task<Product?> GetByIdAsync(string id, CancellationToken ct = default)
    {
        var cacheKey = $"product:{id}";

        // L1: Memory cache (in-process, fastest)
        if (_memoryCache.TryGetValue(cacheKey, out Product? cached))
        {
            _logger.LogDebug("L1 cache hit for {CacheKey}", cacheKey);
            return cached;
        }

        // L2: Distributed cache (Redis)
        var distributed = await _distributedCache.GetStringAsync(cacheKey, ct);
        if (distributed != null)
        {
            _logger.LogDebug("L2 cache hit for {CacheKey}", cacheKey);
            var product = JsonSerializer.Deserialize<Product>(distributed);

            // Populate L1
            _memoryCache.Set(cacheKey, 

Testing Patterns

Unit Tests with xUnit and Moq

public class OrderServiceTests
{
    private readonly Mock<IOrderRepository> _mockRepository;
    private readonly Mock<IStockService> _mockStockService;
    private readonly Mock<IValidator<CreateOrderRequest>> _mockValidator;
    private readonly OrderService _sut; // System Under Test

    public OrderServiceTests()
    {
        _mockRepository = new Mock<IOrderRepository>();
        _mockStockService = new Mock<IStockService>();
        _mockValidator = new Mock<IValidator<CreateOrderRequest>>();

        // Default: validation passes
        _mockValidator
            .Setup(v => v.ValidateAsync(It.IsAny<CreateOrderRequest>(), It.IsAny<CancellationToken>()))
            .ReturnsAsync(new ValidationResult());

        _sut = new OrderService(
            _mockRepository.Object,
            _mockStockService.Object,
            _mockValidator.Object);
    }

    [Fact]
    public async Task CreateOrderAsync_WithValidRequest_ReturnsSuccess()
    {
        // Arrange
        var request = new CreateOrderRequest
        {
            ProductId = "PROD-001",
            Quantity = 5,
            CustomerOrderCode = "ORD-2024-001"
        };

        _mockStockService
            .S