kintsugi-programmer .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;
return options.UseNewEngine
? sp.GetRequiredService<NewPriceCalculator>()
: sp.GetRequiredService<LegacyPriceCalculator>();
});
// Keyed services (.NET 8+)
services.AddKeyedScoped<IPaymentProcessor, StripeProcessor>("stripe");
services.AddKeyedScoped<IPaymentProcessor, PayPalProcessor>("paypal");
return services;
}
}
// Usage with keyed services
public class CheckoutService
{
public CheckoutService(
[FromKeyedServices("stripe")] IPaymentProcessor stripeProcessor)
{
_processor = stripeProcessor;
}
}
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; // NEVER do this
var result2 = GetProductAsync(id).GetAwaiter().GetResult(); // Also bad
// ❌ WRONG: async void (except event handlers)
public async void ProcessOrder() { } // Exceptions are lost
// ❌ WRONG: Unnecessary Task.Run for already async code
await Task.Run(async () => await GetDataAsync()); // Wastes thread
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;
public CatalogService(IOptions<CatalogOptions> options)
{
_options = options.Value;
}
}
// Usage with IOptionsSnapshot (scoped, re-reads on each request)
public class DynamicService
{
private readonly CatalogOptions _options;
public DynamicService(IOptionsSnapshot<CatalogOptions> options)
{
_options = options.Value; // Fresh value per request
}
}
// Usage with IOptionsMonitor (singleton, notified on changes)
public class MonitoredService
{
private CatalogOptions _options;
public MonitoredService(IOptionsMonitor<CatalogOptions> monitor)
{
_options = monitor.CurrentValue;
monitor.OnChange(newOptions => _options = newOptions);
}
}
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(
validation.Errors.First().ErrorMessage,
"VALIDATION_ERROR");
// Business rule check
var stock = await _stockService.CheckAsync(request.ProductId, request.Quantity, ct);
if (!stock.IsAvailable)
return Result<Order>.Failure(
$"Insufficient stock: {stock.Available} available, {request.Quantity} requested",
"INSUFFICIENT_STOCK");
// Create order
var order = await _repository.CreateAsync(request.ToEntity(), ct);
return Result<Order>.Success(order);
}
// Usage in controller/endpoint
app.MapPost("/orders", async (
CreateOrderRequest request,
IOrderService orderService,
CancellationToken ct) =>
{
var result = await orderService.CreateOrderAsync(request, ct);
return result.IsSuccess
? Results.Created($"/orders/{result.Value!.Id}", result.Value)
: Results.BadRequest(new { error = result.Error, code = result.ErrorCode });
});
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
public class ProductRepository : IProductRepository
{
private readonly AppDbContext _context;
public async Task<Product?> GetByIdAsync(string id, CancellationToken ct = default)
{
return await _context.Products
.AsNoTracking()
.FirstOrDefaultAsync(p => p.Id == id, ct);
}
public async Task<IReadOnlyList<Product>> SearchAsync(
ProductSearchCriteria criteria,
CancellationToken ct = default)
{
var query = _context.Products.AsNoTracking();
if (!string.IsNullOrWhiteSpace(criteria.SearchTerm))
query = query.Where(p => EF.Functions.Like(p.Name, $"%{criteria.SearchTerm}%"));
if (criteria.CategoryId.HasValue)
query = query.Where(p => p.CategoryId == criteria.CategoryId);
if (criteria.MinPrice.HasValue)
query = query.Where(p => p.Price >= criteria.MinPrice);
if (criteria.MaxPrice.HasValue)
query = query.Where(p => p.Price <= criteria.MaxPrice);
return await query
.OrderBy(p => p.Name)
.Skip((criteria.Page - 1) * criteria.PageSize)
.Take(criteria.PageSize)
.ToListAsync(ct);
}
}
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)
{
sql.Append(" AND CategoryId = @CategoryId");
parameters.Add("CategoryId", criteria.CategoryId);
}
if (criteria.MinPrice.HasValue)
{
sql.Append(" AND Price >= @MinPrice");
parameters.Add("MinPrice", criteria.MinPrice);
}
if (criteria.MaxPrice.HasValue)
{
sql.Append(" AND Price <= @MaxPrice");
parameters.Add("MaxPrice", criteria.MaxPrice);
}
sql.Append(" ORDER BY Name OFFSET @Offset ROWS FETCH NEXT @PageSize ROWS ONLY");
parameters.Add("Offset", (criteria.Page - 1) * criteria.PageSize);
parameters.Add("PageSize", criteria.PageSize);
var results = await _connection.QueryAsync<Product>(
new CommandDefinition(sql.ToString(), parameters, cancellationToken: ct));
return results.ToList();
}
// Multi-mapping for related data
public async Task<Order?> GetOrderWithItemsAsync(int orderId, CancellationToken ct = default)
{
const string sql = """
SELECT o.*, oi.*, p.*
FROM Orders o
LEFT JOIN OrderItems oi ON o.Id = oi.OrderId
LEFT JOIN Products p ON oi.ProductId = p.Id
WHERE o.Id = @OrderId
""";
var orderDictionary = new Dictionary<int, Order>();
await _connection.QueryAsync<Order, OrderItem, Product, Order>(
new CommandDefinition(sql, new { OrderId = orderId }, cancellationToken: ct),
(order, item, product) =>
{
if (!orderDictionary.TryGetValue(order.Id, out var existingOrder))
{
existingOrder = order;
existingOrder.Items = new List<OrderItem>();
orderDictionary.Add(order.Id, existingOrder);
}
if (item != null)
{
item.Product = product;
existingOrder.Items.Add(item);
}
return existingOrder;
},
splitOn: "Id,Id");
return orderDictionary.Values.FirstOrDefault();
}
}
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, product, MemoryCacheDuration);
return product;
}
// L3: Database
_logger.LogDebug("Cache miss for {CacheKey}, fetching from database", cacheKey);
var fromDb = await _repository.GetByIdAsync(id, ct);
if (fromDb != null)
{
var serialized = JsonSerializer.Serialize(fromDb);
// Populate both caches
await _distributedCache.SetStringAsync(
cacheKey,
serialized,
new DistributedCacheEntryOptions
{
AbsoluteExpirationRelativeToNow = DistributedCacheDuration
},
ct);
_memoryCache.Set(cacheKey, fromDb, MemoryCacheDuration);
}
return fromDb;
}
public async Task InvalidateAsync(string id, CancellationToken ct = default)
{
var cacheKey = $"product:{id}";
_memoryCache.Remove(cacheKey);
await _distributedCache.RemoveAsync(cacheKey, ct);
_logger.LogInformation("Invalidated cache for {CacheKey}", cacheKey);
}
}
// Stale-while-revalidate pattern
public class StaleWhileRevalidateCache<T>
{
private readonly IDistributedCache _cache;
private readonly TimeSpan _freshDuration;
private readonly TimeSpan _staleDuration;
public async Task<T?> GetOrCreateAsync(
string key,
Func<CancellationToken, Task<T>> factory,
CancellationToken ct = default)
{
var cached = await _cache.GetStringAsync(key, ct);
if (cached != null)
{
var entry = JsonSerializer.Deserialize<CacheEntry<T>>(cached)!;
if (entry.IsStale && !entry.IsExpired)
{
// Return stale data immediately, refresh in background
_ = Task.Run(async () =>
{
var fresh = await factory(CancellationToken.None);
await SetAsync(key, fresh, CancellationToken.None);
});
}
if (!entry.IsExpired)
return entry.Value;
}
// Cache miss or expired
var value = await factory(ct);
await SetAsync(key, value, ct);
return value;
}
private record CacheEntry<TValue>(TValue Value, DateTime CreatedAt)
{
public bool IsStale => DateTime.UtcNow - CreatedAt > _freshDuration;
public bool IsExpired => DateTime.UtcNow - CreatedAt > _staleDuration;
}
}
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
.Setup(s => s.CheckAsync("PROD-001", 5, It.IsAny<CancellationToken>()))
.ReturnsAsync(new StockResult { IsAvailable = true, Available = 10 });
_mockRepository
.Setup(r => r.CreateAsync(It.IsAny<Order>(), It.IsAny<CancellationToken>()))
.ReturnsAsync(new Order { Id = 1, CustomerOrderCode = "ORD-2024-001" });
// Act
var result = await _sut.CreateOrderAsync(request);
// Assert
Assert.True(result.IsSuccess);
Assert.NotNull(result.Value);
Assert.Equal(1, result.Value.Id);
_mockRepository.Verify(
r => r.CreateAsync(It.Is<Order>(o => o.CustomerOrderCode == "ORD-2024-001"),
It.IsAny<CancellationToken>()),
Times.Once);
}
[Fact]
public async Task CreateOrderAsync_WithInsufficientStock_ReturnsFailure()
{
// Arrange
var request = new CreateOrderRequest { ProductId = "PROD-001", Quantity = 100 };
_mockStockService
.Setup(s => s.CheckAsync(It.IsAny<string>(), It.IsAny<int>(), It.IsAny<CancellationToken>()))
.ReturnsAsync(new StockResult { IsAvailable = false, Available = 5 });
// Act
var result = await _sut.CreateOrderAsync(request);
// Assert
Assert.False(result.IsSuccess);
Assert.Equal("INSUFFICIENT_STOCK", result.ErrorCode);
Assert.Contains("5 available", result.Error);
_mockRepository.Verify(
r => r.CreateAsync(It.IsAny<Order>(), It.IsAny<CancellationToken>()),
Times.Never);
}
[Theory]
[InlineData(0)]
[InlineData(-1)]
[InlineData(-100)]
public async Task CreateOrderAsync_WithInvalidQuantity_ReturnsValidationError(int quantity)
{
// Arrange
var request = new CreateOrderRequest { ProductId = "PROD-001", Quantity = quantity };
_mockValidator
.Setup(v => v.ValidateAsync(request, It.IsAny<CancellationToken>()))
.ReturnsAsync(new ValidationResult(new[]
{
new ValidationFailure("Quantity", "Quantity must be greater than 0")
}));
// Act
var result = await _sut.CreateOrderAsync(request);
// Assert
Assert.False(result.IsSuccess);
Assert.Equal("VALIDATION_ERROR", result.ErrorCode);
}
}
Integration Tests with WebApplicationFactory
public class ProductsApiTests : IClassFixture<WebApplicationFactory<Program>>
{
private readonly WebApplicationFactory<Program> _factory;
private readonly HttpClient _client;
public ProductsApiTests(WebApplicationFactory<Program> factory)
{
_factory = factory.WithWebHostBuilder(builder =>
{
builder.ConfigureServices(services =>
{
// Replace real database with in-memory
services.RemoveAll<DbContextOptions<AppDbContext>>();
services.AddDbContext<AppDbContext>(options =>
options.UseInMemoryDatabase("TestDb"));
// Replace Redis with memory cache
services.RemoveAll<IDistributedCache>();
services.AddDistributedMemoryCache();
});
});
_client = _factory.CreateClient();
}
[Fact]
public async Task GetProduct_WithValidId_ReturnsProduct()
{
// Arrange
using var scope = _factory.Services.CreateScope();
var context = scope.ServiceProvider.GetRequiredService<AppDbContext>();
context.Products.Add(new Product
{
Id = "TEST-001",
Name = "Test Product",
Price = 99.99m
});
await context.SaveChangesAsync();
// Act
var response = await _client.GetAsync("/api/products/TEST-001");
// Assert
response.EnsureSuccessStatusCode();
var product = await response.Content.ReadFromJsonAsync<Product>();
Assert.Equal("Test Product", product!.Name);
}
[Fact]
public async Task GetProduct_WithInvalidId_Returns404()
{
// Act
var response = await _client.GetAsync("/api/products/NONEXISTENT");
// Assert
Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
}
}
Best Practices
DO
- Use async/await all the way through the call stack
- Inject dependencies through constructor injection
- Use IOptions for typed configuration
- Return Result types instead of throwing exceptions for business logic
- Use CancellationToken in all async methods
- Prefer Dapper for read-heavy, performance-critical queries
- Use EF Core for complex domain models with change tracking
- Cache aggressively with proper invalidation strategies
- Write unit tests for business logic, integration tests for APIs
- Use record types for DTOs and immutable data
DON'T
- Don't block on async with
.Resultor.Wait() - Don't use async void except for event handlers
- Don't catch generic Exception without re-throwing or logging
- Don't hardcode configuration values
- Don't expose EF entities directly in APIs (use DTOs)
- Don't forget
AsNoTracking()for read-only queries - Don't ignore CancellationToken parameters
- Don't create
new HttpClient()manually (use IHttpClientFactory) - Don't mix sync and async code unnecessarily
- Don't skip validation at API boundaries
Common Pitfalls
- N+1 Queries: Use
.Include()or explicit joins - Memory Leaks: Dispose IDisposable resources, use
using - Deadlocks: Don't mix sync and async, use ConfigureAwait(false) in libraries
- Over-fetching: Select only needed columns, use projections
- Missing Indexes: Check query plans, add indexes for common filters
- Timeout Issues: Configure appropriate timeouts for HTTP clients
- Cache Stampede: Use distributed locks for cache population