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dotnet-backend-patterns
Master C#/.NET backend development patterns for building robust
# .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/
āāā Domain/ # Core business logic (no dependencies)
ā āāā Entities/
ā āāā Interfaces/
ā āāā Exceptions/
ā āāā ValueObjects/
āāā Application/ # Use cases, DTOs, validation
ā āāā Services/
ā āāā DTOs/
ā āāā Validators/
ā āāā Interfaces/
āāā Infrastructure/ # External implementations
ā āāā Data/ # EF Core, Dapper repositories
ā āāā Caching/ # Redis, Memory cache
ā āāā External/ # HTTP clients, third-party APIs
ā āāā DependencyInjection/ # Service registration
āāā Api/ # Entry point
āāā Controllers/ # Or MinimalAPI endpoints
āāā Middleware/
āāā Filters/
āāā Program.cs
```
### 2. Dependency Injection Patterns
```csharp
// 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
```csharp
// ā
CORRECT: Async all the way down
public async Task<Product> GetProductAsync(string id, CancellationToken ct = default)
{
return await _repository.GetByIdAsync(id, ct);
}
// ā
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);
}
// ā
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);
}
// ā
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));
}
// ā WRONG: Blocking on async (deadlock risk)
var result = GetProductAsync(id).Result; // NEVER do this
var result2 = GetProductAsync(id).GetAwaiter().GetResult(); // Also bad
// ā WRONG: async void (except event handlers)
public async void ProcessOrder() { } // Exceptions are lost
// ā WRONG: Unnecessary Task.Run for already async code
await Task.Run(async () => await GetDataAsync()); // Wastes thread
```
### 4. Configuration with IOptions
```csharp
// 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)
```csharp
// 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
```csharp
// 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
```csharp
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
```csharp
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
```csharp
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<
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