dotnet-backend-patterns

.NET バックエンド開発パターン

最新のベストプラクティス（2024/2025）を用いて、本番品質の API、MCP サーバー、エンタープライズバックエンドを構築するための C#/.NET パターンを習得します。

このスキルを使用する場面

• 新しい .NET Web API または MCP サーバーを開発するとき
• 品質とパフォーマンスの観点で C# コードをレビューするとき
• 依存性注入を用いてサービスアーキテクチャを設計するとき
• Redis を用いたキャッシュ戦略を実装するとき
• 単体テストと統合テストを書くとき
• EF Core または Dapper を用いてデータベースアクセスを最適化するとき
• IOptions パターンを用いてアプリケーションを構成するとき
• エラーを処理し、レジリエンスパターンを実装するとき

中核となる概念

1. プロジェクト構造（クリーンアーキテクチャ）

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. 依存性注入パターン

// 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 パターン

// ✅ 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. IOptions による構成

// 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 パターン（フロー制御における例外の回避）

// 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 });
});

データアクセスパターン

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);
    }
}

パフォーマンスのための Dapper

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();
    }
}

キャッシュパターン

Redis によるマルチレベルキャッシュ

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;
    }
}

テストパターン

xUnit と Moq による単体テスト

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);
    }
}

WebApplicationFactory による統合テスト

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);
    }
}

ベストプラクティス

すべきこと

1. コールスタック全体を通して async/await を使用する
2. 依存関係を注入する際はコンストラクターインジェクションを使用する
3. 型付き構成には IOptions を使用する
4. ビジネスロジックでは例外をスローする代わりに Result 型を返す
5. すべての非同期メソッドで CancellationToken を使用する
6. 読み取りが多くパフォーマンスが重要なクエリには Dapper を優先する
7. 変更追跡を伴う複雑なドメインモデルには EF Core を使用する
8. 適切な無効化戦略とともに 積極的にキャッシュする
9. ビジネスロジックには 単体テストを、API には統合テストを書く
10. DTO と不変データには record 型を使用する

すべきでないこと

1. .Result や .Wait() で 非同期処理をブロックしない
2. イベントハンドラーを除いて async void を使用しない
3. 再スローやログ出力なしに 汎用 Exception をキャッチしない
4. 構成値を ハードコードしない
5. API で EF エンティティを直接公開しない（DTO を使用する）
6. 読み取り専用クエリで AsNoTracking() を 忘れない
7. CancellationToken パラメーターを 無視しない
8. new HttpClient() を手動で 作成しない（IHttpClientFactory を使用する）
9. 不必要に同期コードと非同期コードを 混在させない
10. API 境界での検証を 省略しない

よくある落とし穴

• N+1 クエリ: .Include() または明示的な結合を使用する
• メモリリーク: IDisposable リソースを破棄し、using を使用する
• デッドロック: 同期コードと非同期コードを混在させず、ライブラリでは ConfigureAwait(false) を使用する
• 過剰フェッチ: 必要な列のみを選択し、プロジェクションを使用する
• インデックスの欠落: クエリプランを確認し、よく使われるフィルターにインデックスを追加する
• タイムアウトの問題: HTTP クライアントに適切なタイムアウトを構成する
• キャッシュスタンピード: キャッシュの生成には分散ロックを使用する