[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"skill:01M6F1CSW9AG969ZX5JXZ0Y2JM":3,"skill-same:01M6F1CSW9AG969ZX5JXZ0Y2JM":107},{"skillNo":4,"name":5,"nameZh":6,"description":7,"descriptionZh":6,"repositoryFullName":8,"ownerLogin":9,"ownerAvatarUrl":10,"ownerAccountNo":11,"ownerDisplayName":12,"repositoryNo":13,"starsCount":14,"licenseSpdxId":15,"licenseFileUrl":6,"sourceHtmlUrl":16,"categories":17,"keywords":34,"publishedAt":55,"linkOnly":56,"markdownContent":57,"markdownBody":58,"frontMatter":59,"refName":60,"commitSha":61,"skillDirectory":62,"forksCount":63,"repositoryHtmlUrl":64,"repositoryHomepageUrl":65,"ownerHtmlUrl":66,"ownerBlogUrl":6,"ownerDescription":67,"updatedAt":68,"install":69,"resources":82},"01M6F1CSW9AG969ZX5JXZ0Y2JM","dotnet-backend-patterns",null,"Master C#\u002F.NET backend development patterns for building robust APIs, MCP servers, and enterprise applications. Covers async\u002Fawait, dependency injection, Entity Framework Core, Dapper, configuration, caching, and testing with xUnit. Use when developing .NET backends, reviewing C# code, or designing API architectures.","ComeOnOliver\u002Fskillshub","ComeOnOliver","https:\u002F\u002Favatars.githubusercontent.com\u002Fu\u002F43215937?v=4","01M6E5JNFECJAAVFE0W7SMWWD2","Haoran Shu","01M6E5JNNDQS7AJZDYFQ8TPZCK",67,"MIT","https:\u002F\u002Fgithub.com\u002FComeOnOliver\u002Fskillshub\u002Fblob\u002Fmain\u002Fskills\u002Faiskillstore\u002Fmarketplace\u002Fwshobson\u002Fdotnet-backend-patterns\u002FSKILL.md",[18,23,26,29],{"categoryNo":19,"categoryName":20,"taxonomyType":21,"taxonomyTypeDesc":22},"01JBSEEDCAT000000000000008","测试与质量","SCENARIO","使用场景",{"categoryNo":24,"categoryName":25,"taxonomyType":21,"taxonomyTypeDesc":22},"01JBSEEDCAT000000000000010","后端与接口",{"categoryNo":27,"categoryName":28,"taxonomyType":21,"taxonomyTypeDesc":22},"01JBSEEDCAT000000000000023","Skill与智能体开发",{"categoryNo":30,"categoryName":31,"taxonomyType":32,"taxonomyTypeDesc":33},"01JBSEEDCAT000000000000030","开发工程师","PROFESSION","适用职业",[35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54],"dotnet","patterns","dotnet backend patterns","dotnet backend","backend patterns","backend","architectures","testing","dependency injection","caching","master","framework core","dependency","api","injection","dapper","framework","code","development","MCP","2026-11-10 10:12:30",false,"---\nname: dotnet-backend-patterns\ndescription: Master C#\u002F.NET backend development patterns for building robust APIs, MCP servers, and enterprise applications. Covers async\u002Fawait, dependency injection, Entity Framework Core, Dapper, configuration, caching, and testing with xUnit. Use when developing .NET backends, reviewing C# code, or designing API architectures.\n---\n\n# .NET Backend Development Patterns\n\nMaster C#\u002F.NET patterns for building production-grade APIs, MCP servers, and enterprise backends with modern best practices (2024\u002F2025).\n\n## When to Use This Skill\n\n- Developing new .NET Web APIs or MCP servers\n- Reviewing C# code for quality and performance\n- Designing service architectures with dependency injection\n- Implementing caching strategies with Redis\n- Writing unit and integration tests\n- Optimizing database access with EF Core or Dapper\n- Configuring applications with IOptions pattern\n- Handling errors and implementing resilience patterns\n\n## Core Concepts\n\n### 1. Project Structure (Clean Architecture)\n\n```\nsrc\u002F\n├── Domain\u002F                     # Core business logic (no dependencies)\n│   ├── Entities\u002F\n│   ├── Interfaces\u002F\n│   ├── Exceptions\u002F\n│   └── ValueObjects\u002F\n├── Application\u002F                # Use cases, DTOs, validation\n│   ├── Services\u002F\n│   ├── DTOs\u002F\n│   ├── Validators\u002F\n│   └── Interfaces\u002F\n├── Infrastructure\u002F             # External implementations\n│   ├── Data\u002F                   # EF Core, Dapper repositories\n│   ├── Caching\u002F                # Redis, Memory cache\n│   ├── External\u002F               # HTTP clients, third-party APIs\n│   └── DependencyInjection\u002F    # Service registration\n└── Api\u002F                        # Entry point\n    ├── Controllers\u002F            # Or MinimalAPI endpoints\n    ├── Middleware\u002F\n    ├── Filters\u002F\n    └── Program.cs\n```\n\n### 2. Dependency Injection Patterns\n\n```csharp\n\u002F\u002F Service registration by lifetime\npublic static class ServiceCollectionExtensions\n{\n    public static IServiceCollection AddApplicationServices(\n        this IServiceCollection services,\n        IConfiguration configuration)\n    {\n        \u002F\u002F Scoped: One instance per HTTP request\n        services.AddScoped\u003CIProductService, ProductService>();\n        services.AddScoped\u003CIOrderService, OrderService>();\n        \n        \u002F\u002F Singleton: One instance for app lifetime\n        services.AddSingleton\u003CICacheService, RedisCacheService>();\n        services.AddSingleton\u003CIConnectionMultiplexer>(_ =>\n            ConnectionMultiplexer.Connect(configuration[\"Redis:Connection\"]!));\n        \n        \u002F\u002F Transient: New instance every time\n        services.AddTransient\u003CIValidator\u003CCreateOrderRequest>, CreateOrderValidator>();\n        \n        \u002F\u002F Options pattern for configuration\n        services.Configure\u003CCatalogOptions>(configuration.GetSection(\"Catalog\"));\n        services.Configure\u003CRedisOptions>(configuration.GetSection(\"Redis\"));\n        \n        \u002F\u002F Factory pattern for conditional creation\n        services.AddScoped\u003CIPriceCalculator>(sp =>\n        {\n            var options = sp.GetRequiredService\u003CIOptions\u003CPricingOptions>>().Value;\n            return options.UseNewEngine\n                ? sp.GetRequiredService\u003CNewPriceCalculator>()\n                : sp.GetRequiredService\u003CLegacyPriceCalculator>();\n        });\n        \n        \u002F\u002F Keyed services (.NET 8+)\n        services.AddKeyedScoped\u003CIPaymentProcessor, StripeProcessor>(\"stripe\");\n        services.AddKeyedScoped\u003CIPaymentProcessor, PayPalProcessor>(\"paypal\");\n        \n        return services;\n    }\n}\n\n\u002F\u002F Usage with keyed services\npublic class CheckoutService\n{\n    public CheckoutService(\n        [FromKeyedServices(\"stripe\")] IPaymentProcessor stripeProcessor)\n    {\n        _processor = stripeProcessor;\n    }\n}\n```\n\n### 3. Async\u002FAwait Patterns\n\n```csharp\n\u002F\u002F ✅ CORRECT: Async all the way down\npublic async Task\u003CProduct> GetProductAsync(string id, CancellationToken ct = default)\n{\n    return await _repository.GetByIdAsync(id, ct);\n}\n\n\u002F\u002F ✅ CORRECT: Parallel execution with WhenAll\npublic async Task\u003C(Stock, Price)> GetStockAndPriceAsync(\n    string productId, \n    CancellationToken ct = default)\n{\n    var stockTask = _stockService.GetAsync(productId, ct);\n    var priceTask = _priceService.GetAsync(productId, ct);\n    \n    await Task.WhenAll(stockTask, priceTask);\n    \n    return (await stockTask, await priceTask);\n}\n\n\u002F\u002F ✅ CORRECT: ConfigureAwait in libraries\npublic async Task\u003CT> LibraryMethodAsync\u003CT>(CancellationToken ct = default)\n{\n    var result = await _httpClient.GetAsync(url, ct).ConfigureAwait(false);\n    return await result.Content.ReadFromJsonAsync\u003CT>(ct).ConfigureAwait(false);\n}\n\n\u002F\u002F ✅ CORRECT: ValueTask for hot paths with caching\npublic ValueTask\u003CProduct?> GetCachedProductAsync(string id)\n{\n    if (_cache.TryGetValue(id, out Product? product))\n        return ValueTask.FromResult(product);\n    \n    return new ValueTask\u003CProduct?>(GetFromDatabaseAsync(id));\n}\n\n\u002F\u002F ❌ WRONG: Blocking on async (deadlock risk)\nvar result = GetProductAsync(id).Result;  \u002F\u002F NEVER do this\nvar result2 = GetProductAsync(id).GetAwaiter().GetResult(); \u002F\u002F Also bad\n\n\u002F\u002F ❌ WRONG: async void (except event handlers)\npublic async void ProcessOrder() { }  \u002F\u002F Exceptions are lost\n\n\u002F\u002F ❌ WRONG: Unnecessary Task.Run for already async code\nawait Task.Run(async () => await GetDataAsync());  \u002F\u002F Wastes thread\n```\n\n### 4. Configuration with IOptions\n\n```csharp\n\u002F\u002F Configuration classes\npublic class CatalogOptions\n{\n    public const string SectionName = \"Catalog\";\n    \n    public int DefaultPageSize { get; set; } = 50;\n    public int MaxPageSize { get; set; } = 200;\n    public TimeSpan CacheDuration { get; set; } = TimeSpan.FromMinutes(15);\n    public bool EnableEnrichment { get; set; } = true;\n}\n\npublic class RedisOptions\n{\n    public const string SectionName = \"Redis\";\n    \n    public string Connection { get; set; } = \"localhost:6379\";\n    public string KeyPrefix { get; set; } = \"mcp:\";\n    public int Database { get; set; } = 0;\n}\n\n\u002F\u002F appsettings.json\n{\n    \"Catalog\": {\n        \"DefaultPageSize\": 50,\n        \"MaxPageSize\": 200,\n        \"CacheDuration\": \"00:15:00\",\n        \"EnableEnrichment\": true\n    },\n    \"Redis\": {\n        \"Connection\": \"localhost:6379\",\n        \"KeyPrefix\": \"mcp:\",\n        \"Database\": 0\n    }\n}\n\n\u002F\u002F Registration\nservices.Configure\u003CCatalogOptions>(configuration.GetSection(CatalogOptions.SectionName));\nservices.Configure\u003CRedisOptions>(configuration.GetSection(RedisOptions.SectionName));\n\n\u002F\u002F Usage with IOptions (singleton, read once at startup)\npublic class CatalogService\n{\n    private readonly CatalogOptions _options;\n    \n    public CatalogService(IOptions\u003CCatalogOptions> options)\n    {\n        _options = options.Value;\n    }\n}\n\n\u002F\u002F Usage with IOptionsSnapshot (scoped, re-reads on each request)\npublic class DynamicService\n{\n    private readonly CatalogOptions _options;\n    \n    public DynamicService(IOptionsSnapshot\u003CCatalogOptions> options)\n    {\n        _options = options.Value;  \u002F\u002F Fresh value per request\n    }\n}\n\n\u002F\u002F Usage with IOptionsMonitor (singleton, notified on changes)\npublic class MonitoredService\n{\n    private CatalogOptions _options;\n    \n    public MonitoredService(IOptionsMonitor\u003CCatalogOptions> monitor)\n    {\n        _options = monitor.CurrentValue;\n        monitor.OnChange(newOptions => _options = newOptions);\n    }\n}\n```\n\n### 5. Result Pattern (Avoiding Exceptions for Flow Control)\n\n```csharp\n\u002F\u002F Generic Result type\npublic class Result\u003CT>\n{\n    public bool IsSuccess { get; }\n    public T? Value { get; }\n    public string? Error { get; }\n    public string? ErrorCode { get; }\n    \n    private Result(bool isSuccess, T? value, string? error, string? errorCode)\n    {\n        IsSuccess = isSuccess;\n        Value = value;\n        Error = error;\n        ErrorCode = errorCode;\n    }\n    \n    public static Result\u003CT> Success(T value) => new(true, value, null, null);\n    public static Result\u003CT> Failure(string error, string? code = null) => new(false, default, error, code);\n    \n    public Result\u003CTNew> Map\u003CTNew>(Func\u003CT, TNew> mapper) =>\n        IsSuccess ? Result\u003CTNew>.Success(mapper(Value!)) : Result\u003CTNew>.Failure(Error!, ErrorCode);\n    \n    public async Task\u003CResult\u003CTNew>> MapAsync\u003CTNew>(Func\u003CT, Task\u003CTNew>> mapper) =>\n        IsSuccess ? Result\u003CTNew>.Success(await mapper(Value!)) : Result\u003CTNew>.Failure(Error!, ErrorCode);\n}\n\n\u002F\u002F Usage in service\npublic async Task\u003CResult\u003COrder>> CreateOrderAsync(CreateOrderRequest request, CancellationToken ct)\n{\n    \u002F\u002F Validation\n    var validation = await _validator.ValidateAsync(request, ct);\n    if (!validation.IsValid)\n        return Result\u003COrder>.Failure(\n            validation.Errors.First().ErrorMessage, \n            \"VALIDATION_ERROR\");\n    \n    \u002F\u002F Business rule check\n    var stock = await _stockService.CheckAsync(request.ProductId, request.Quantity, ct);\n    if (!stock.IsAvailable)\n        return Result\u003COrder>.Failure(\n            $\"Insufficient stock: {stock.Available} available, {request.Quantity} requested\",\n            \"INSUFFICIENT_STOCK\");\n    \n    \u002F\u002F Create order\n    var order = await _repository.CreateAsync(request.ToEntity(), ct);\n    \n    return Result\u003COrder>.Success(order);\n}\n\n\u002F\u002F Usage in controller\u002Fendpoint\napp.MapPost(\"\u002Forders\", async (\n    CreateOrderRequest request,\n    IOrderService orderService,\n    CancellationToken ct) =>\n{\n    var result = await orderService.CreateOrderAsync(request, ct);\n    \n    return result.IsSuccess\n        ? Results.Created($\"\u002Forders\u002F{result.Value!.Id}\", result.Value)\n        : Results.BadRequest(new { error = result.Error, code = result.ErrorCode });\n});\n```\n\n## Data Access Patterns\n\n### Entity Framework Core\n\n```csharp\n\u002F\u002F DbContext configuration\npublic class AppDbContext : DbContext\n{\n    public DbSet\u003CProduct> Products => Set\u003CProduct>();\n    public DbSet\u003COrder> Orders => Set\u003COrder>();\n    \n    protected override void OnModelCreating(ModelBuilder modelBuilder)\n    {\n        \u002F\u002F Apply all configurations from assembly\n        modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);\n        \n        \u002F\u002F Global query filters\n        modelBuilder.Entity\u003CProduct>().HasQueryFilter(p => !p.IsDeleted);\n    }\n}\n\n\u002F\u002F Entity configuration\npublic class ProductConfiguration : IEntityTypeConfiguration\u003CProduct>\n{\n    public void Configure(EntityTypeBuilder\u003CProduct> builder)\n    {\n        builder.ToTable(\"Products\");\n        \n        builder.HasKey(p => p.Id);\n        builder.Property(p => p.Id).HasMaxLength(40);\n        builder.Property(p => p.Name).HasMaxLength(200).IsRequired();\n        builder.Property(p => p.Price).HasPrecision(18, 2);\n        \n        builder.HasIndex(p => p.Sku).IsUnique();\n        builder.HasIndex(p => new { p.CategoryId, p.Name });\n        \n        builder.HasMany(p => p.OrderItems)\n            .WithOne(oi => oi.Product)\n            .HasForeignKey(oi => oi.ProductId);\n    }\n}\n\n\u002F\u002F Repository with EF Core\npublic class ProductRepository : IProductRepository\n{\n    private readonly AppDbContext _context;\n    \n    public async Task\u003CProduct?> GetByIdAsync(string id, CancellationToken ct = default)\n    {\n        return await _context.Products\n            .AsNoTracking()\n            .FirstOrDefaultAsync(p => p.Id == id, ct);\n    }\n    \n    public async Task\u003CIReadOnlyList\u003CProduct>> SearchAsync(\n        ProductSearchCriteria criteria,\n        CancellationToken ct = default)\n    {\n        var query = _context.Products.AsNoTracking();\n        \n        if (!string.IsNullOrWhiteSpace(criteria.SearchTerm))\n            query = query.Where(p => EF.Functions.Like(p.Name, $\"%{criteria.SearchTerm}%\"));\n        \n        if (criteria.CategoryId.HasValue)\n            query = query.Where(p => p.CategoryId == criteria.CategoryId);\n        \n        if (criteria.MinPrice.HasValue)\n            query = query.Where(p => p.Price >= criteria.MinPrice);\n        \n        if (criteria.MaxPrice.HasValue)\n            query = query.Where(p => p.Price \u003C= criteria.MaxPrice);\n        \n        return await query\n            .OrderBy(p => p.Name)\n            .Skip((criteria.Page - 1) * criteria.PageSize)\n            .Take(criteria.PageSize)\n            .ToListAsync(ct);\n    }\n}\n```\n\n### Dapper for Performance\n\n```csharp\npublic class DapperProductRepository : IProductRepository\n{\n    private readonly IDbConnection _connection;\n    \n    public async Task\u003CProduct?> GetByIdAsync(string id, CancellationToken ct = default)\n    {\n        const string sql = \"\"\"\n            SELECT Id, Name, Sku, Price, CategoryId, Stock, CreatedAt\n            FROM Products\n            WHERE Id = @Id AND IsDeleted = 0\n            \"\"\";\n        \n        return await _connection.QueryFirstOrDefaultAsync\u003CProduct>(\n            new CommandDefinition(sql, new { Id = id }, cancellationToken: ct));\n    }\n    \n    public async Task\u003CIReadOnlyList\u003CProduct>> SearchAsync(\n        ProductSearchCriteria criteria,\n        CancellationToken ct = default)\n    {\n        var sql = new StringBuilder(\"\"\"\n            SELECT Id, Name, Sku, Price, CategoryId, Stock, CreatedAt\n            FROM Products\n            WHERE IsDeleted = 0\n            \"\"\");\n        \n        var parameters = new DynamicParameters();\n        \n        if (!string.IsNullOrWhiteSpace(criteria.SearchTerm))\n        {\n            sql.Append(\" AND Name LIKE @SearchTerm\");\n            parameters.Add(\"SearchTerm\", $\"%{criteria.SearchTerm}%\");\n        }\n        \n        if (criteria.CategoryId.HasValue)\n        {\n            sql.Append(\" AND CategoryId = @CategoryId\");\n            parameters.Add(\"CategoryId\", criteria.CategoryId);\n        }\n        \n        if (criteria.MinPrice.HasValue)\n        {\n            sql.Append(\" AND Price >= @MinPrice\");\n            parameters.Add(\"MinPrice\", criteria.MinPrice);\n        }\n        \n        if (criteria.MaxPrice.HasValue)\n        {\n            sql.Append(\" AND Price \u003C= @MaxPrice\");\n            parameters.Add(\"MaxPrice\", criteria.MaxPrice);\n        }\n        \n        sql.Append(\" ORDER BY Name OFFSET @Offset ROWS FETCH NEXT @PageSize ROWS ONLY\");\n        parameters.Add(\"Offset\", (criteria.Page - 1) * criteria.PageSize);\n        parameters.Add(\"PageSize\", criteria.PageSize);\n        \n        var results = await _connection.QueryAsync\u003CProduct>(\n            new CommandDefinition(sql.ToString(), parameters, cancellationToken: ct));\n        \n        return results.ToList();\n    }\n    \n    \u002F\u002F Multi-mapping for related data\n    public async Task\u003COrder?> GetOrderWithItemsAsync(int orderId, CancellationToken ct = default)\n    {\n        const string sql = \"\"\"\n            SELECT o.*, oi.*, p.*\n            FROM Orders o\n            LEFT JOIN OrderItems oi ON o.Id = oi.OrderId\n            LEFT JOIN Products p ON oi.ProductId = p.Id\n            WHERE o.Id = @OrderId\n            \"\"\";\n        \n        var orderDictionary = new Dictionary\u003Cint, Order>();\n        \n        await _connection.QueryAsync\u003COrder, OrderItem, Product, Order>(\n            new CommandDefinition(sql, new { OrderId = orderId }, cancellationToken: ct),\n            (order, item, product) =>\n            {\n                if (!orderDictionary.TryGetValue(order.Id, out var existingOrder))\n                {\n                    existingOrder = order;\n                    existingOrder.Items = new List\u003COrderItem>();\n                    orderDictionary.Add(order.Id, existingOrder);\n                }\n                \n                if (item != null)\n                {\n                    item.Product = product;\n                    existingOrder.Items.Add(item);\n                }\n                \n                return existingOrder;\n            },\n            splitOn: \"Id,Id\");\n        \n        return orderDictionary.Values.FirstOrDefault();\n    }\n}\n```\n\n## Caching Patterns\n\n### Multi-Level Cache with Redis\n\n```csharp\npublic class CachedProductService : IProductService\n{\n    private readonly IProductRepository _repository;\n    private readonly IMemoryCache _memoryCache;\n    private readonly IDistributedCache _distributedCache;\n    private readonly ILogger\u003CCachedProductService> _logger;\n    \n    private static readonly TimeSpan MemoryCacheDuration = TimeSpan.FromMinutes(1);\n    private static readonly TimeSpan DistributedCacheDuration = TimeSpan.FromMinutes(15);\n    \n    public async Task\u003CProduct?> GetByIdAsync(string id, CancellationToken ct = default)\n    {\n        var cacheKey = $\"product:{id}\";\n        \n        \u002F\u002F L1: Memory cache (in-process, fastest)\n        if (_memoryCache.TryGetValue(cacheKey, out Product? cached))\n        {\n            _logger.LogDebug(\"L1 cache hit for {CacheKey}\", cacheKey);\n            return cached;\n        }\n        \n        \u002F\u002F L2: Distributed cache (Redis)\n        var distributed = await _distributedCache.GetStringAsync(cacheKey, ct);\n        if (distributed != null)\n        {\n            _logger.LogDebug(\"L2 cache hit for {CacheKey}\", cacheKey);\n            var product = JsonSerializer.Deserialize\u003CProduct>(distributed);\n            \n            \u002F\u002F Populate L1\n            _memoryCache.Set(cacheKey, product, MemoryCacheDuration);\n            return product;\n        }\n        \n        \u002F\u002F L3: Database\n        _logger.LogDebug(\"Cache miss for {CacheKey}, fetching from database\", cacheKey);\n        var fromDb = await _repository.GetByIdAsync(id, ct);\n        \n        if (fromDb != null)\n        {\n            var serialized = JsonSerializer.Serialize(fromDb);\n            \n            \u002F\u002F Populate both caches\n            await _distributedCache.SetStringAsync(\n                cacheKey,\n                serialized,\n                new DistributedCacheEntryOptions\n                {\n                    AbsoluteExpirationRelativeToNow = DistributedCacheDuration\n                },\n                ct);\n            \n            _memoryCache.Set(cacheKey, fromDb, MemoryCacheDuration);\n        }\n        \n        return fromDb;\n    }\n    \n    public async Task InvalidateAsync(string id, CancellationToken ct = default)\n    {\n        var cacheKey = $\"product:{id}\";\n        \n        _memoryCache.Remove(cacheKey);\n        await _distributedCache.RemoveAsync(cacheKey, ct);\n        \n        _logger.LogInformation(\"Invalidated cache for {CacheKey}\", cacheKey);\n    }\n}\n\n\u002F\u002F Stale-while-revalidate pattern\npublic class StaleWhileRevalidateCache\u003CT>\n{\n    private readonly IDistributedCache _cache;\n    private readonly TimeSpan _freshDuration;\n    private readonly TimeSpan _staleDuration;\n    \n    public async Task\u003CT?> GetOrCreateAsync(\n        string key,\n        Func\u003CCancellationToken, Task\u003CT>> factory,\n        CancellationToken ct = default)\n    {\n        var cached = await _cache.GetStringAsync(key, ct);\n        \n        if (cached != null)\n        {\n            var entry = JsonSerializer.Deserialize\u003CCacheEntry\u003CT>>(cached)!;\n            \n            if (entry.IsStale && !entry.IsExpired)\n            {\n                \u002F\u002F Return stale data immediately, refresh in background\n                _ = Task.Run(async () =>\n                {\n                    var fresh = await factory(CancellationToken.None);\n                    await SetAsync(key, fresh, CancellationToken.None);\n                });\n            }\n            \n            if (!entry.IsExpired)\n                return entry.Value;\n        }\n        \n        \u002F\u002F Cache miss or expired\n        var value = await factory(ct);\n        await SetAsync(key, value, ct);\n        return value;\n    }\n    \n    private record CacheEntry\u003CTValue>(TValue Value, DateTime CreatedAt)\n    {\n        public bool IsStale => DateTime.UtcNow - CreatedAt > _freshDuration;\n        public bool IsExpired => DateTime.UtcNow - CreatedAt > _staleDuration;\n    }\n}\n```\n\n## Testing Patterns\n\n### Unit Tests with xUnit and Moq\n\n```csharp\npublic class OrderServiceTests\n{\n    private readonly Mock\u003CIOrderRepository> _mockRepository;\n    private readonly Mock\u003CIStockService> _mockStockService;\n    private readonly Mock\u003CIValidator\u003CCreateOrderRequest>> _mockValidator;\n    private readonly OrderService _sut; \u002F\u002F System Under Test\n    \n    public OrderServiceTests()\n    {\n        _mockRepository = new Mock\u003CIOrderRepository>();\n        _mockStockService = new Mock\u003CIStockService>();\n        _mockValidator = new Mock\u003CIValidator\u003CCreateOrderRequest>>();\n        \n        \u002F\u002F Default: validation passes\n        _mockValidator\n            .Setup(v => v.ValidateAsync(It.IsAny\u003CCreateOrderRequest>(), It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new ValidationResult());\n        \n        _sut = new OrderService(\n            _mockRepository.Object,\n            _mockStockService.Object,\n            _mockValidator.Object);\n    }\n    \n    [Fact]\n    public async Task CreateOrderAsync_WithValidRequest_ReturnsSuccess()\n    {\n        \u002F\u002F Arrange\n        var request = new CreateOrderRequest\n        {\n            ProductId = \"PROD-001\",\n            Quantity = 5,\n            CustomerOrderCode = \"ORD-2024-001\"\n        };\n        \n        _mockStockService\n            .Setup(s => s.CheckAsync(\"PROD-001\", 5, It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new StockResult { IsAvailable = true, Available = 10 });\n        \n        _mockRepository\n            .Setup(r => r.CreateAsync(It.IsAny\u003COrder>(), It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new Order { Id = 1, CustomerOrderCode = \"ORD-2024-001\" });\n        \n        \u002F\u002F Act\n        var result = await _sut.CreateOrderAsync(request);\n        \n        \u002F\u002F Assert\n        Assert.True(result.IsSuccess);\n        Assert.NotNull(result.Value);\n        Assert.Equal(1, result.Value.Id);\n        \n        _mockRepository.Verify(\n            r => r.CreateAsync(It.Is\u003COrder>(o => o.CustomerOrderCode == \"ORD-2024-001\"), \n            It.IsAny\u003CCancellationToken>()), \n            Times.Once);\n    }\n    \n    [Fact]\n    public async Task CreateOrderAsync_WithInsufficientStock_ReturnsFailure()\n    {\n        \u002F\u002F Arrange\n        var request = new CreateOrderRequest { ProductId = \"PROD-001\", Quantity = 100 };\n        \n        _mockStockService\n            .Setup(s => s.CheckAsync(It.IsAny\u003Cstring>(), It.IsAny\u003Cint>(), It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new StockResult { IsAvailable = false, Available = 5 });\n        \n        \u002F\u002F Act\n        var result = await _sut.CreateOrderAsync(request);\n        \n        \u002F\u002F Assert\n        Assert.False(result.IsSuccess);\n        Assert.Equal(\"INSUFFICIENT_STOCK\", result.ErrorCode);\n        Assert.Contains(\"5 available\", result.Error);\n        \n        _mockRepository.Verify(\n            r => r.CreateAsync(It.IsAny\u003COrder>(), It.IsAny\u003CCancellationToken>()), \n            Times.Never);\n    }\n    \n    [Theory]\n    [InlineData(0)]\n    [InlineData(-1)]\n    [InlineData(-100)]\n    public async Task CreateOrderAsync_WithInvalidQuantity_ReturnsValidationError(int quantity)\n    {\n        \u002F\u002F Arrange\n        var request = new CreateOrderRequest { ProductId = \"PROD-001\", Quantity = quantity };\n        \n        _mockValidator\n            .Setup(v => v.ValidateAsync(request, It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new ValidationResult(new[]\n            {\n                new ValidationFailure(\"Quantity\", \"Quantity must be greater than 0\")\n            }));\n        \n        \u002F\u002F Act\n        var result = await _sut.CreateOrderAsync(request);\n        \n        \u002F\u002F Assert\n        Assert.False(result.IsSuccess);\n        Assert.Equal(\"VALIDATION_ERROR\", result.ErrorCode);\n    }\n}\n```\n\n### Integration Tests with WebApplicationFactory\n\n```csharp\npublic class ProductsApiTests : IClassFixture\u003CWebApplicationFactory\u003CProgram>>\n{\n    private readonly WebApplicationFactory\u003CProgram> _factory;\n    private readonly HttpClient _client;\n    \n    public ProductsApiTests(WebApplicationFactory\u003CProgram> factory)\n    {\n        _factory = factory.WithWebHostBuilder(builder =>\n        {\n            builder.ConfigureServices(services =>\n            {\n                \u002F\u002F Replace real database with in-memory\n                services.RemoveAll\u003CDbContextOptions\u003CAppDbContext>>();\n                services.AddDbContext\u003CAppDbContext>(options =>\n                    options.UseInMemoryDatabase(\"TestDb\"));\n                \n                \u002F\u002F Replace Redis with memory cache\n                services.RemoveAll\u003CIDistributedCache>();\n                services.AddDistributedMemoryCache();\n            });\n        });\n        \n        _client = _factory.CreateClient();\n    }\n    \n    [Fact]\n    public async Task GetProduct_WithValidId_ReturnsProduct()\n    {\n        \u002F\u002F Arrange\n        using var scope = _factory.Services.CreateScope();\n        var context = scope.ServiceProvider.GetRequiredService\u003CAppDbContext>();\n        \n        context.Products.Add(new Product\n        {\n            Id = \"TEST-001\",\n            Name = \"Test Product\",\n            Price = 99.99m\n        });\n        await context.SaveChangesAsync();\n        \n        \u002F\u002F Act\n        var response = await _client.GetAsync(\"\u002Fapi\u002Fproducts\u002FTEST-001\");\n        \n        \u002F\u002F Assert\n        response.EnsureSuccessStatusCode();\n        var product = await response.Content.ReadFromJsonAsync\u003CProduct>();\n        Assert.Equal(\"Test Product\", product!.Name);\n    }\n    \n    [Fact]\n    public async Task GetProduct_WithInvalidId_Returns404()\n    {\n        \u002F\u002F Act\n        var response = await _client.GetAsync(\"\u002Fapi\u002Fproducts\u002FNONEXISTENT\");\n        \n        \u002F\u002F Assert\n        Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);\n    }\n}\n```\n\n## Best Practices\n\n### DO\n1. **Use async\u002Fawait** all the way through the call stack\n2. **Inject dependencies** through constructor injection\n3. **Use IOptions\u003CT>** for typed configuration\n4. **Return Result types** instead of throwing exceptions for business logic\n5. **Use CancellationToken** in all async methods\n6. **Prefer Dapper** for read-heavy, performance-critical queries\n7. **Use EF Core** for complex domain models with change tracking\n8. **Cache aggressively** with proper invalidation strategies\n9. **Write unit tests** for business logic, integration tests for APIs\n10. **Use record types** for DTOs and immutable data\n\n### DON'T\n1. **Don't block on async** with `.Result` or `.Wait()`\n2. **Don't use async void** except for event handlers\n3. **Don't catch generic Exception** without re-throwing or logging\n4. **Don't hardcode** configuration values\n5. **Don't expose EF entities** directly in APIs (use DTOs)\n6. **Don't forget** `AsNoTracking()` for read-only queries\n7. **Don't ignore** CancellationToken parameters\n8. **Don't create** `new HttpClient()` manually (use IHttpClientFactory)\n9. **Don't mix** sync and async code unnecessarily\n10. **Don't skip** validation at API boundaries\n\n## Common Pitfalls\n\n- **N+1 Queries**: Use `.Include()` or explicit joins\n- **Memory Leaks**: Dispose IDisposable resources, use `using`\n- **Deadlocks**: Don't mix sync and async, use ConfigureAwait(false) in libraries\n- **Over-fetching**: Select only needed columns, use projections\n- **Missing Indexes**: Check query plans, add indexes for common filters\n- **Timeout Issues**: Configure appropriate timeouts for HTTP clients\n- **Cache Stampede**: Use distributed locks for cache population\n\n## Resources\n\n- **assets\u002Fservice-template.cs**: Complete service implementation template\n- **assets\u002Frepository-template.cs**: Repository pattern implementation\n- **references\u002Fef-core-best-practices.md**: EF Core optimization guide\n- **references\u002Fdapper-patterns.md**: Advanced Dapper usage patterns\n","# .NET Backend Development Patterns\n\nMaster C#\u002F.NET patterns for building production-grade APIs, MCP servers, and enterprise backends with modern best practices (2024\u002F2025).\n\n## When to Use This Skill\n\n- Developing new .NET Web APIs or MCP servers\n- Reviewing C# code for quality and performance\n- Designing service architectures with dependency injection\n- Implementing caching strategies with Redis\n- Writing unit and integration tests\n- Optimizing database access with EF Core or Dapper\n- Configuring applications with IOptions pattern\n- Handling errors and implementing resilience patterns\n\n## Core Concepts\n\n### 1. Project Structure (Clean Architecture)\n\n```\nsrc\u002F\n├── Domain\u002F                     # Core business logic (no dependencies)\n│   ├── Entities\u002F\n│   ├── Interfaces\u002F\n│   ├── Exceptions\u002F\n│   └── ValueObjects\u002F\n├── Application\u002F                # Use cases, DTOs, validation\n│   ├── Services\u002F\n│   ├── DTOs\u002F\n│   ├── Validators\u002F\n│   └── Interfaces\u002F\n├── Infrastructure\u002F             # External implementations\n│   ├── Data\u002F                   # EF Core, Dapper repositories\n│   ├── Caching\u002F                # Redis, Memory cache\n│   ├── External\u002F               # HTTP clients, third-party APIs\n│   └── DependencyInjection\u002F    # Service registration\n└── Api\u002F                        # Entry point\n    ├── Controllers\u002F            # Or MinimalAPI endpoints\n    ├── Middleware\u002F\n    ├── Filters\u002F\n    └── Program.cs\n```\n\n### 2. Dependency Injection Patterns\n\n```csharp\n\u002F\u002F Service registration by lifetime\npublic static class ServiceCollectionExtensions\n{\n    public static IServiceCollection AddApplicationServices(\n        this IServiceCollection services,\n        IConfiguration configuration)\n    {\n        \u002F\u002F Scoped: One instance per HTTP request\n        services.AddScoped\u003CIProductService, ProductService>();\n        services.AddScoped\u003CIOrderService, OrderService>();\n        \n        \u002F\u002F Singleton: One instance for app lifetime\n        services.AddSingleton\u003CICacheService, RedisCacheService>();\n        services.AddSingleton\u003CIConnectionMultiplexer>(_ =>\n            ConnectionMultiplexer.Connect(configuration[\"Redis:Connection\"]!));\n        \n        \u002F\u002F Transient: New instance every time\n        services.AddTransient\u003CIValidator\u003CCreateOrderRequest>, CreateOrderValidator>();\n        \n        \u002F\u002F Options pattern for configuration\n        services.Configure\u003CCatalogOptions>(configuration.GetSection(\"Catalog\"));\n        services.Configure\u003CRedisOptions>(configuration.GetSection(\"Redis\"));\n        \n        \u002F\u002F Factory pattern for conditional creation\n        services.AddScoped\u003CIPriceCalculator>(sp =>\n        {\n            var options = sp.GetRequiredService\u003CIOptions\u003CPricingOptions>>().Value;\n            return options.UseNewEngine\n                ? sp.GetRequiredService\u003CNewPriceCalculator>()\n                : sp.GetRequiredService\u003CLegacyPriceCalculator>();\n        });\n        \n        \u002F\u002F Keyed services (.NET 8+)\n        services.AddKeyedScoped\u003CIPaymentProcessor, StripeProcessor>(\"stripe\");\n        services.AddKeyedScoped\u003CIPaymentProcessor, PayPalProcessor>(\"paypal\");\n        \n        return services;\n    }\n}\n\n\u002F\u002F Usage with keyed services\npublic class CheckoutService\n{\n    public CheckoutService(\n        [FromKeyedServices(\"stripe\")] IPaymentProcessor stripeProcessor)\n    {\n        _processor = stripeProcessor;\n    }\n}\n```\n\n### 3. Async\u002FAwait Patterns\n\n```csharp\n\u002F\u002F ✅ CORRECT: Async all the way down\npublic async Task\u003CProduct> GetProductAsync(string id, CancellationToken ct = default)\n{\n    return await _repository.GetByIdAsync(id, ct);\n}\n\n\u002F\u002F ✅ CORRECT: Parallel execution with WhenAll\npublic async Task\u003C(Stock, Price)> GetStockAndPriceAsync(\n    string productId, \n    CancellationToken ct = default)\n{\n    var stockTask = _stockService.GetAsync(productId, ct);\n    var priceTask = _priceService.GetAsync(productId, ct);\n    \n    await Task.WhenAll(stockTask, priceTask);\n    \n    return (await stockTask, await priceTask);\n}\n\n\u002F\u002F ✅ CORRECT: ConfigureAwait in libraries\npublic async Task\u003CT> LibraryMethodAsync\u003CT>(CancellationToken ct = default)\n{\n    var result = await _httpClient.GetAsync(url, ct).ConfigureAwait(false);\n    return await result.Content.ReadFromJsonAsync\u003CT>(ct).ConfigureAwait(false);\n}\n\n\u002F\u002F ✅ CORRECT: ValueTask for hot paths with caching\npublic ValueTask\u003CProduct?> GetCachedProductAsync(string id)\n{\n    if (_cache.TryGetValue(id, out Product? product))\n        return ValueTask.FromResult(product);\n    \n    return new ValueTask\u003CProduct?>(GetFromDatabaseAsync(id));\n}\n\n\u002F\u002F ❌ WRONG: Blocking on async (deadlock risk)\nvar result = GetProductAsync(id).Result;  \u002F\u002F NEVER do this\nvar result2 = GetProductAsync(id).GetAwaiter().GetResult(); \u002F\u002F Also bad\n\n\u002F\u002F ❌ WRONG: async void (except event handlers)\npublic async void ProcessOrder() { }  \u002F\u002F Exceptions are lost\n\n\u002F\u002F ❌ WRONG: Unnecessary Task.Run for already async code\nawait Task.Run(async () => await GetDataAsync());  \u002F\u002F Wastes thread\n```\n\n### 4. Configuration with IOptions\n\n```csharp\n\u002F\u002F Configuration classes\npublic class CatalogOptions\n{\n    public const string SectionName = \"Catalog\";\n    \n    public int DefaultPageSize { get; set; } = 50;\n    public int MaxPageSize { get; set; } = 200;\n    public TimeSpan CacheDuration { get; set; } = TimeSpan.FromMinutes(15);\n    public bool EnableEnrichment { get; set; } = true;\n}\n\npublic class RedisOptions\n{\n    public const string SectionName = \"Redis\";\n    \n    public string Connection { get; set; } = \"localhost:6379\";\n    public string KeyPrefix { get; set; } = \"mcp:\";\n    public int Database { get; set; } = 0;\n}\n\n\u002F\u002F appsettings.json\n{\n    \"Catalog\": {\n        \"DefaultPageSize\": 50,\n        \"MaxPageSize\": 200,\n        \"CacheDuration\": \"00:15:00\",\n        \"EnableEnrichment\": true\n    },\n    \"Redis\": {\n        \"Connection\": \"localhost:6379\",\n        \"KeyPrefix\": \"mcp:\",\n        \"Database\": 0\n    }\n}\n\n\u002F\u002F Registration\nservices.Configure\u003CCatalogOptions>(configuration.GetSection(CatalogOptions.SectionName));\nservices.Configure\u003CRedisOptions>(configuration.GetSection(RedisOptions.SectionName));\n\n\u002F\u002F Usage with IOptions (singleton, read once at startup)\npublic class CatalogService\n{\n    private readonly CatalogOptions _options;\n    \n    public CatalogService(IOptions\u003CCatalogOptions> options)\n    {\n        _options = options.Value;\n    }\n}\n\n\u002F\u002F Usage with IOptionsSnapshot (scoped, re-reads on each request)\npublic class DynamicService\n{\n    private readonly CatalogOptions _options;\n    \n    public DynamicService(IOptionsSnapshot\u003CCatalogOptions> options)\n    {\n        _options = options.Value;  \u002F\u002F Fresh value per request\n    }\n}\n\n\u002F\u002F Usage with IOptionsMonitor (singleton, notified on changes)\npublic class MonitoredService\n{\n    private CatalogOptions _options;\n    \n    public MonitoredService(IOptionsMonitor\u003CCatalogOptions> monitor)\n    {\n        _options = monitor.CurrentValue;\n        monitor.OnChange(newOptions => _options = newOptions);\n    }\n}\n```\n\n### 5. Result Pattern (Avoiding Exceptions for Flow Control)\n\n```csharp\n\u002F\u002F Generic Result type\npublic class Result\u003CT>\n{\n    public bool IsSuccess { get; }\n    public T? Value { get; }\n    public string? Error { get; }\n    public string? ErrorCode { get; }\n    \n    private Result(bool isSuccess, T? value, string? error, string? errorCode)\n    {\n        IsSuccess = isSuccess;\n        Value = value;\n        Error = error;\n        ErrorCode = errorCode;\n    }\n    \n    public static Result\u003CT> Success(T value) => new(true, value, null, null);\n    public static Result\u003CT> Failure(string error, string? code = null) => new(false, default, error, code);\n    \n    public Result\u003CTNew> Map\u003CTNew>(Func\u003CT, TNew> mapper) =>\n        IsSuccess ? Result\u003CTNew>.Success(mapper(Value!)) : Result\u003CTNew>.Failure(Error!, ErrorCode);\n    \n    public async Task\u003CResult\u003CTNew>> MapAsync\u003CTNew>(Func\u003CT, Task\u003CTNew>> mapper) =>\n        IsSuccess ? Result\u003CTNew>.Success(await mapper(Value!)) : Result\u003CTNew>.Failure(Error!, ErrorCode);\n}\n\n\u002F\u002F Usage in service\npublic async Task\u003CResult\u003COrder>> CreateOrderAsync(CreateOrderRequest request, CancellationToken ct)\n{\n    \u002F\u002F Validation\n    var validation = await _validator.ValidateAsync(request, ct);\n    if (!validation.IsValid)\n        return Result\u003COrder>.Failure(\n            validation.Errors.First().ErrorMessage, \n            \"VALIDATION_ERROR\");\n    \n    \u002F\u002F Business rule check\n    var stock = await _stockService.CheckAsync(request.ProductId, request.Quantity, ct);\n    if (!stock.IsAvailable)\n        return Result\u003COrder>.Failure(\n            $\"Insufficient stock: {stock.Available} available, {request.Quantity} requested\",\n            \"INSUFFICIENT_STOCK\");\n    \n    \u002F\u002F Create order\n    var order = await _repository.CreateAsync(request.ToEntity(), ct);\n    \n    return Result\u003COrder>.Success(order);\n}\n\n\u002F\u002F Usage in controller\u002Fendpoint\napp.MapPost(\"\u002Forders\", async (\n    CreateOrderRequest request,\n    IOrderService orderService,\n    CancellationToken ct) =>\n{\n    var result = await orderService.CreateOrderAsync(request, ct);\n    \n    return result.IsSuccess\n        ? Results.Created($\"\u002Forders\u002F{result.Value!.Id}\", result.Value)\n        : Results.BadRequest(new { error = result.Error, code = result.ErrorCode });\n});\n```\n\n## Data Access Patterns\n\n### Entity Framework Core\n\n```csharp\n\u002F\u002F DbContext configuration\npublic class AppDbContext : DbContext\n{\n    public DbSet\u003CProduct> Products => Set\u003CProduct>();\n    public DbSet\u003COrder> Orders => Set\u003COrder>();\n    \n    protected override void OnModelCreating(ModelBuilder modelBuilder)\n    {\n        \u002F\u002F Apply all configurations from assembly\n        modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);\n        \n        \u002F\u002F Global query filters\n        modelBuilder.Entity\u003CProduct>().HasQueryFilter(p => !p.IsDeleted);\n    }\n}\n\n\u002F\u002F Entity configuration\npublic class ProductConfiguration : IEntityTypeConfiguration\u003CProduct>\n{\n    public void Configure(EntityTypeBuilder\u003CProduct> builder)\n    {\n        builder.ToTable(\"Products\");\n        \n        builder.HasKey(p => p.Id);\n        builder.Property(p => p.Id).HasMaxLength(40);\n        builder.Property(p => p.Name).HasMaxLength(200).IsRequired();\n        builder.Property(p => p.Price).HasPrecision(18, 2);\n        \n        builder.HasIndex(p => p.Sku).IsUnique();\n        builder.HasIndex(p => new { p.CategoryId, p.Name });\n        \n        builder.HasMany(p => p.OrderItems)\n            .WithOne(oi => oi.Product)\n            .HasForeignKey(oi => oi.ProductId);\n    }\n}\n\n\u002F\u002F Repository with EF Core\npublic class ProductRepository : IProductRepository\n{\n    private readonly AppDbContext _context;\n    \n    public async Task\u003CProduct?> GetByIdAsync(string id, CancellationToken ct = default)\n    {\n        return await _context.Products\n            .AsNoTracking()\n            .FirstOrDefaultAsync(p => p.Id == id, ct);\n    }\n    \n    public async Task\u003CIReadOnlyList\u003CProduct>> SearchAsync(\n        ProductSearchCriteria criteria,\n        CancellationToken ct = default)\n    {\n        var query = _context.Products.AsNoTracking();\n        \n        if (!string.IsNullOrWhiteSpace(criteria.SearchTerm))\n            query = query.Where(p => EF.Functions.Like(p.Name, $\"%{criteria.SearchTerm}%\"));\n        \n        if (criteria.CategoryId.HasValue)\n            query = query.Where(p => p.CategoryId == criteria.CategoryId);\n        \n        if (criteria.MinPrice.HasValue)\n            query = query.Where(p => p.Price >= criteria.MinPrice);\n        \n        if (criteria.MaxPrice.HasValue)\n            query = query.Where(p => p.Price \u003C= criteria.MaxPrice);\n        \n        return await query\n            .OrderBy(p => p.Name)\n            .Skip((criteria.Page - 1) * criteria.PageSize)\n            .Take(criteria.PageSize)\n            .ToListAsync(ct);\n    }\n}\n```\n\n### Dapper for Performance\n\n```csharp\npublic class DapperProductRepository : IProductRepository\n{\n    private readonly IDbConnection _connection;\n    \n    public async Task\u003CProduct?> GetByIdAsync(string id, CancellationToken ct = default)\n    {\n        const string sql = \"\"\"\n            SELECT Id, Name, Sku, Price, CategoryId, Stock, CreatedAt\n            FROM Products\n            WHERE Id = @Id AND IsDeleted = 0\n            \"\"\";\n        \n        return await _connection.QueryFirstOrDefaultAsync\u003CProduct>(\n            new CommandDefinition(sql, new { Id = id }, cancellationToken: ct));\n    }\n    \n    public async Task\u003CIReadOnlyList\u003CProduct>> SearchAsync(\n        ProductSearchCriteria criteria,\n        CancellationToken ct = default)\n    {\n        var sql = new StringBuilder(\"\"\"\n            SELECT Id, Name, Sku, Price, CategoryId, Stock, CreatedAt\n            FROM Products\n            WHERE IsDeleted = 0\n            \"\"\");\n        \n        var parameters = new DynamicParameters();\n        \n        if (!string.IsNullOrWhiteSpace(criteria.SearchTerm))\n        {\n            sql.Append(\" AND Name LIKE @SearchTerm\");\n            parameters.Add(\"SearchTerm\", $\"%{criteria.SearchTerm}%\");\n        }\n        \n        if (criteria.CategoryId.HasValue)\n        {\n            sql.Append(\" AND CategoryId = @CategoryId\");\n            parameters.Add(\"CategoryId\", criteria.CategoryId);\n        }\n        \n        if (criteria.MinPrice.HasValue)\n        {\n            sql.Append(\" AND Price >= @MinPrice\");\n            parameters.Add(\"MinPrice\", criteria.MinPrice);\n        }\n        \n        if (criteria.MaxPrice.HasValue)\n        {\n            sql.Append(\" AND Price \u003C= @MaxPrice\");\n            parameters.Add(\"MaxPrice\", criteria.MaxPrice);\n        }\n        \n        sql.Append(\" ORDER BY Name OFFSET @Offset ROWS FETCH NEXT @PageSize ROWS ONLY\");\n        parameters.Add(\"Offset\", (criteria.Page - 1) * criteria.PageSize);\n        parameters.Add(\"PageSize\", criteria.PageSize);\n        \n        var results = await _connection.QueryAsync\u003CProduct>(\n            new CommandDefinition(sql.ToString(), parameters, cancellationToken: ct));\n        \n        return results.ToList();\n    }\n    \n    \u002F\u002F Multi-mapping for related data\n    public async Task\u003COrder?> GetOrderWithItemsAsync(int orderId, CancellationToken ct = default)\n    {\n        const string sql = \"\"\"\n            SELECT o.*, oi.*, p.*\n            FROM Orders o\n            LEFT JOIN OrderItems oi ON o.Id = oi.OrderId\n            LEFT JOIN Products p ON oi.ProductId = p.Id\n            WHERE o.Id = @OrderId\n            \"\"\";\n        \n        var orderDictionary = new Dictionary\u003Cint, Order>();\n        \n        await _connection.QueryAsync\u003COrder, OrderItem, Product, Order>(\n            new CommandDefinition(sql, new { OrderId = orderId }, cancellationToken: ct),\n            (order, item, product) =>\n            {\n                if (!orderDictionary.TryGetValue(order.Id, out var existingOrder))\n                {\n                    existingOrder = order;\n                    existingOrder.Items = new List\u003COrderItem>();\n                    orderDictionary.Add(order.Id, existingOrder);\n                }\n                \n                if (item != null)\n                {\n                    item.Product = product;\n                    existingOrder.Items.Add(item);\n                }\n                \n                return existingOrder;\n            },\n            splitOn: \"Id,Id\");\n        \n        return orderDictionary.Values.FirstOrDefault();\n    }\n}\n```\n\n## Caching Patterns\n\n### Multi-Level Cache with Redis\n\n```csharp\npublic class CachedProductService : IProductService\n{\n    private readonly IProductRepository _repository;\n    private readonly IMemoryCache _memoryCache;\n    private readonly IDistributedCache _distributedCache;\n    private readonly ILogger\u003CCachedProductService> _logger;\n    \n    private static readonly TimeSpan MemoryCacheDuration = TimeSpan.FromMinutes(1);\n    private static readonly TimeSpan DistributedCacheDuration = TimeSpan.FromMinutes(15);\n    \n    public async Task\u003CProduct?> GetByIdAsync(string id, CancellationToken ct = default)\n    {\n        var cacheKey = $\"product:{id}\";\n        \n        \u002F\u002F L1: Memory cache (in-process, fastest)\n        if (_memoryCache.TryGetValue(cacheKey, out Product? cached))\n        {\n            _logger.LogDebug(\"L1 cache hit for {CacheKey}\", cacheKey);\n            return cached;\n        }\n        \n        \u002F\u002F L2: Distributed cache (Redis)\n        var distributed = await _distributedCache.GetStringAsync(cacheKey, ct);\n        if (distributed != null)\n        {\n            _logger.LogDebug(\"L2 cache hit for {CacheKey}\", cacheKey);\n            var product = JsonSerializer.Deserialize\u003CProduct>(distributed);\n            \n            \u002F\u002F Populate L1\n            _memoryCache.Set(cacheKey, product, MemoryCacheDuration);\n            return product;\n        }\n        \n        \u002F\u002F L3: Database\n        _logger.LogDebug(\"Cache miss for {CacheKey}, fetching from database\", cacheKey);\n        var fromDb = await _repository.GetByIdAsync(id, ct);\n        \n        if (fromDb != null)\n        {\n            var serialized = JsonSerializer.Serialize(fromDb);\n            \n            \u002F\u002F Populate both caches\n            await _distributedCache.SetStringAsync(\n                cacheKey,\n                serialized,\n                new DistributedCacheEntryOptions\n                {\n                    AbsoluteExpirationRelativeToNow = DistributedCacheDuration\n                },\n                ct);\n            \n            _memoryCache.Set(cacheKey, fromDb, MemoryCacheDuration);\n        }\n        \n        return fromDb;\n    }\n    \n    public async Task InvalidateAsync(string id, CancellationToken ct = default)\n    {\n        var cacheKey = $\"product:{id}\";\n        \n        _memoryCache.Remove(cacheKey);\n        await _distributedCache.RemoveAsync(cacheKey, ct);\n        \n        _logger.LogInformation(\"Invalidated cache for {CacheKey}\", cacheKey);\n    }\n}\n\n\u002F\u002F Stale-while-revalidate pattern\npublic class StaleWhileRevalidateCache\u003CT>\n{\n    private readonly IDistributedCache _cache;\n    private readonly TimeSpan _freshDuration;\n    private readonly TimeSpan _staleDuration;\n    \n    public async Task\u003CT?> GetOrCreateAsync(\n        string key,\n        Func\u003CCancellationToken, Task\u003CT>> factory,\n        CancellationToken ct = default)\n    {\n        var cached = await _cache.GetStringAsync(key, ct);\n        \n        if (cached != null)\n        {\n            var entry = JsonSerializer.Deserialize\u003CCacheEntry\u003CT>>(cached)!;\n            \n            if (entry.IsStale && !entry.IsExpired)\n            {\n                \u002F\u002F Return stale data immediately, refresh in background\n                _ = Task.Run(async () =>\n                {\n                    var fresh = await factory(CancellationToken.None);\n                    await SetAsync(key, fresh, CancellationToken.None);\n                });\n            }\n            \n            if (!entry.IsExpired)\n                return entry.Value;\n        }\n        \n        \u002F\u002F Cache miss or expired\n        var value = await factory(ct);\n        await SetAsync(key, value, ct);\n        return value;\n    }\n    \n    private record CacheEntry\u003CTValue>(TValue Value, DateTime CreatedAt)\n    {\n        public bool IsStale => DateTime.UtcNow - CreatedAt > _freshDuration;\n        public bool IsExpired => DateTime.UtcNow - CreatedAt > _staleDuration;\n    }\n}\n```\n\n## Testing Patterns\n\n### Unit Tests with xUnit and Moq\n\n```csharp\npublic class OrderServiceTests\n{\n    private readonly Mock\u003CIOrderRepository> _mockRepository;\n    private readonly Mock\u003CIStockService> _mockStockService;\n    private readonly Mock\u003CIValidator\u003CCreateOrderRequest>> _mockValidator;\n    private readonly OrderService _sut; \u002F\u002F System Under Test\n    \n    public OrderServiceTests()\n    {\n        _mockRepository = new Mock\u003CIOrderRepository>();\n        _mockStockService = new Mock\u003CIStockService>();\n        _mockValidator = new Mock\u003CIValidator\u003CCreateOrderRequest>>();\n        \n        \u002F\u002F Default: validation passes\n        _mockValidator\n            .Setup(v => v.ValidateAsync(It.IsAny\u003CCreateOrderRequest>(), It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new ValidationResult());\n        \n        _sut = new OrderService(\n            _mockRepository.Object,\n            _mockStockService.Object,\n            _mockValidator.Object);\n    }\n    \n    [Fact]\n    public async Task CreateOrderAsync_WithValidRequest_ReturnsSuccess()\n    {\n        \u002F\u002F Arrange\n        var request = new CreateOrderRequest\n        {\n            ProductId = \"PROD-001\",\n            Quantity = 5,\n            CustomerOrderCode = \"ORD-2024-001\"\n        };\n        \n        _mockStockService\n            .Setup(s => s.CheckAsync(\"PROD-001\", 5, It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new StockResult { IsAvailable = true, Available = 10 });\n        \n        _mockRepository\n            .Setup(r => r.CreateAsync(It.IsAny\u003COrder>(), It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new Order { Id = 1, CustomerOrderCode = \"ORD-2024-001\" });\n        \n        \u002F\u002F Act\n        var result = await _sut.CreateOrderAsync(request);\n        \n        \u002F\u002F Assert\n        Assert.True(result.IsSuccess);\n        Assert.NotNull(result.Value);\n        Assert.Equal(1, result.Value.Id);\n        \n        _mockRepository.Verify(\n            r => r.CreateAsync(It.Is\u003COrder>(o => o.CustomerOrderCode == \"ORD-2024-001\"), \n            It.IsAny\u003CCancellationToken>()), \n            Times.Once);\n    }\n    \n    [Fact]\n    public async Task CreateOrderAsync_WithInsufficientStock_ReturnsFailure()\n    {\n        \u002F\u002F Arrange\n        var request = new CreateOrderRequest { ProductId = \"PROD-001\", Quantity = 100 };\n        \n        _mockStockService\n            .Setup(s => s.CheckAsync(It.IsAny\u003Cstring>(), It.IsAny\u003Cint>(), It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new StockResult { IsAvailable = false, Available = 5 });\n        \n        \u002F\u002F Act\n        var result = await _sut.CreateOrderAsync(request);\n        \n        \u002F\u002F Assert\n        Assert.False(result.IsSuccess);\n        Assert.Equal(\"INSUFFICIENT_STOCK\", result.ErrorCode);\n        Assert.Contains(\"5 available\", result.Error);\n        \n        _mockRepository.Verify(\n            r => r.CreateAsync(It.IsAny\u003COrder>(), It.IsAny\u003CCancellationToken>()), \n            Times.Never);\n    }\n    \n    [Theory]\n    [InlineData(0)]\n    [InlineData(-1)]\n    [InlineData(-100)]\n    public async Task CreateOrderAsync_WithInvalidQuantity_ReturnsValidationError(int quantity)\n    {\n        \u002F\u002F Arrange\n        var request = new CreateOrderRequest { ProductId = \"PROD-001\", Quantity = quantity };\n        \n        _mockValidator\n            .Setup(v => v.ValidateAsync(request, It.IsAny\u003CCancellationToken>()))\n            .ReturnsAsync(new ValidationResult(new[]\n            {\n                new ValidationFailure(\"Quantity\", \"Quantity must be greater than 0\")\n            }));\n        \n        \u002F\u002F Act\n        var result = await _sut.CreateOrderAsync(request);\n        \n        \u002F\u002F Assert\n        Assert.False(result.IsSuccess);\n        Assert.Equal(\"VALIDATION_ERROR\", result.ErrorCode);\n    }\n}\n```\n\n### Integration Tests with WebApplicationFactory\n\n```csharp\npublic class ProductsApiTests : IClassFixture\u003CWebApplicationFactory\u003CProgram>>\n{\n    private readonly WebApplicationFactory\u003CProgram> _factory;\n    private readonly HttpClient _client;\n    \n    public ProductsApiTests(WebApplicationFactory\u003CProgram> factory)\n    {\n        _factory = factory.WithWebHostBuilder(builder =>\n        {\n            builder.ConfigureServices(services =>\n            {\n                \u002F\u002F Replace real database with in-memory\n                services.RemoveAll\u003CDbContextOptions\u003CAppDbContext>>();\n                services.AddDbContext\u003CAppDbContext>(options =>\n                    options.UseInMemoryDatabase(\"TestDb\"));\n                \n                \u002F\u002F Replace Redis with memory cache\n                services.RemoveAll\u003CIDistributedCache>();\n                services.AddDistributedMemoryCache();\n            });\n        });\n        \n        _client = _factory.CreateClient();\n    }\n    \n    [Fact]\n    public async Task GetProduct_WithValidId_ReturnsProduct()\n    {\n        \u002F\u002F Arrange\n        using var scope = _factory.Services.CreateScope();\n        var context = scope.ServiceProvider.GetRequiredService\u003CAppDbContext>();\n        \n        context.Products.Add(new Product\n        {\n            Id = \"TEST-001\",\n            Name = \"Test Product\",\n            Price = 99.99m\n        });\n        await context.SaveChangesAsync();\n        \n        \u002F\u002F Act\n        var response = await _client.GetAsync(\"\u002Fapi\u002Fproducts\u002FTEST-001\");\n        \n        \u002F\u002F Assert\n        response.EnsureSuccessStatusCode();\n        var product = await response.Content.ReadFromJsonAsync\u003CProduct>();\n        Assert.Equal(\"Test Product\", product!.Name);\n    }\n    \n    [Fact]\n    public async Task GetProduct_WithInvalidId_Returns404()\n    {\n        \u002F\u002F Act\n        var response = await _client.GetAsync(\"\u002Fapi\u002Fproducts\u002FNONEXISTENT\");\n        \n        \u002F\u002F Assert\n        Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);\n    }\n}\n```\n\n## Best Practices\n\n### DO\n1. **Use async\u002Fawait** all the way through the call stack\n2. **Inject dependencies** through constructor injection\n3. **Use IOptions\u003CT>** for typed configuration\n4. **Return Result types** instead of throwing exceptions for business logic\n5. **Use CancellationToken** in all async methods\n6. **Prefer Dapper** for read-heavy, performance-critical queries\n7. **Use EF Core** for complex domain models with change tracking\n8. **Cache aggressively** with proper invalidation strategies\n9. **Write unit tests** for business logic, integration tests for APIs\n10. **Use record types** for DTOs and immutable data\n\n### DON'T\n1. **Don't block on async** with `.Result` or `.Wait()`\n2. **Don't use async void** except for event handlers\n3. **Don't catch generic Exception** without re-throwing or logging\n4. **Don't hardcode** configuration values\n5. **Don't expose EF entities** directly in APIs (use DTOs)\n6. **Don't forget** `AsNoTracking()` for read-only queries\n7. **Don't ignore** CancellationToken parameters\n8. **Don't create** `new HttpClient()` manually (use IHttpClientFactory)\n9. **Don't mix** sync and async code unnecessarily\n10. **Don't skip** validation at API boundaries\n\n## Common Pitfalls\n\n- **N+1 Queries**: Use `.Include()` or explicit joins\n- **Memory Leaks**: Dispose IDisposable resources, use `using`\n- **Deadlocks**: Don't mix sync and async, use ConfigureAwait(false) in libraries\n- **Over-fetching**: Select only needed columns, use projections\n- **Missing Indexes**: Check query plans, add indexes for common filters\n- **Timeout Issues**: Configure appropriate timeouts for HTTP clients\n- **Cache Stampede**: Use distributed locks for cache population\n\n## Resources\n\n- **assets\u002Fservice-template.cs**: Complete service implementation template\n- **assets\u002Frepository-template.cs**: Repository pattern implementation\n- **references\u002Fef-core-best-practices.md**: EF Core optimization guide\n- **references\u002Fdapper-patterns.md**: Advanced Dapper usage patterns\n",{"name":5,"description":7},"main","def8531e65114c0fca8fb8551c1871ee0eed705c","skills\u002Faiskillstore\u002Fmarketplace\u002Fwshobson\u002Fdotnet-backend-patterns",24,"https:\u002F\u002Fgithub.com\u002FComeOnOliver\u002Fskillshub","https:\u002F\u002Fskillshub.wtf","https:\u002F\u002Fgithub.com\u002FComeOnOliver","AI & Blockchain Engineer\r\n","2026-06-24 22:07:19",{"source":8,"skillName":5,"commands":70,"prompt":80,"zipAvailable":81},[71,74,77],{"tool":72,"command":73},"npx","npx skills add ComeOnOliver\u002Fskillshub --skill dotnet-backend-patterns",{"tool":75,"command":76},"bunx","bunx skills add 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