name: dotnet-backend-patterns type: reference description: "Provides .NET and ASP.NET Core patterns for REST APIs, Entity Framework, dependency injection, and middleware. Use when working with C# files (.cs, .csproj) or when the user mentions .NET, ASP.NET Core, C#, or Entity Framework." paths: ["**/.cs", "**/.csproj", "/*.sln", "/appsettings*.json"] effort: 3 allowed-tools: Read, Glob, Grep user-invocable: true when_to_use: "When building C#/.NET backend APIs, MCP servers, or enterprise applications with Entity Framework or Dapper"

.NET Backend Development Patterns

C#/.NET patterns for production-grade APIs, MCP servers, and enterprise backends.

API Structure (Minimal API + Controllers)

```csharp // Program.cs - Minimal API var builder = WebApplication.CreateBuilder(args); builder.Services.AddDbContext<AppDbContext>(o => o.UseNpgsql(connStr)); builder.Services.AddScoped<IOrderService, OrderService>();

var app = builder.Build(); app.MapGet("/orders/{id}", async (int id, IOrderService svc) => await svc.GetByIdAsync(id) is { } order ? Results.Ok(order) : Results.NotFound()); ```

Dependency Injection Patterns

```csharp // Register services builder.Services.AddScoped<IPaymentService, StripePaymentService>(); builder.Services.AddSingleton<ICacheService, RedisCacheService>(); builder.Services.AddHttpClient<IApiClient, ExternalApiClient>(client => { client.BaseAddress = new Uri("https://api.external.com"); }); ```

Lifetime guide: Singleton (stateless/cache), Scoped (per-request), Transient (stateless utility).

Entity Framework Core

```csharp // DbContext with conventions public class AppDbContext : DbContext { public DbSet<Order> Orders => Set<Order>(); protected override void OnModelCreating(ModelBuilder builder) { builder.Entity<Order>().HasIndex(o => o.UserId); builder.Entity<Order>().Property(o => o.Total).HasPrecision(18, 2); } } ```

Performance tips:

• Use .AsNoTracking() for read-only queries
• Avoid N+1 with .Include() or projection
• Use compiled queries for hot paths

Middleware Pipeline

```csharp app.UseMiddleware<RequestLoggingMiddleware>(); app.UseAuthentication(); app.UseAuthorization(); app.UseRateLimiter(); app.MapControllers(); ```

Error Handling

```csharp // Global exception handler app.UseExceptionHandler(err => err.Run(async context => { var exception = context.Features.Get<IExceptionHandlerFeature>()?.Error; var (status, message) = exception switch { NotFoundException => (404, exception.Message), UnauthorizedAccessException => (403, "Forbidden"), _ => (500, "Internal server error") }; context.Response.StatusCode = status; await context.Response.WriteAsJsonAsync(new { error = message }); })); ```

Configuration (IOptions pattern)

```csharp builder.Services.Configure<StripeSettings>( builder.Configuration.GetSection("Stripe"));

// Usage public class PaymentService(IOptions<StripeSettings> opts) { private readonly string _key = opts.Value.SecretKey; } ```

Testing

```csharp // Integration test with WebApplicationFactory public class OrderApiTests : IClassFixture<WebApplicationFactory<Program>> { private readonly HttpClient _client; [Fact] public async Task GetOrder_ReturnsOk() { var response = await _client.GetAsync("/orders/1"); response.StatusCode.Should().Be(HttpStatusCode.OK); } } ```

Related Skills

• backend-architect — architecture decisions
• database-architect — schema design
• drizzle-orm-expert — Node.js ORM alternative