src/
├── Features/
│   ├── Orders/
│   │   ├── CreateOrder.cs
│   │   └── GetOrder.cs
│   └── Users/
│       └── RegisterUser.cs
├── Core/
│   ├── Behaviors/
│   │   └── ValidationBehavior.cs
│   └── Contracts/
│       └── ApiResult.cs
└── Program.cs

public record CreateOrderRequest(
    Guid UserId,
    List<OrderItemDto> Items,
    ShippingAddressDto Address);

public record OrderItemDto(Guid ProductId, int Quantity);
public record ShippingAddressDto(string Street, string City, string ZipCode);

public record CreateOrderResponse(Guid OrderId, DateTime CreatedAt);

public class CreateOrderHandler : IRequestHandler<CreateOrderRequest, CreateOrderResponse>
{
    private readonly IOrderRepository _repository;
    private readonly IPaymentGateway _payment;

    public CreateOrderHandler(IOrderRepository repository, IPaymentGateway payment)
    {
        _repository = repository;
        _payment = payment;
    }

    public async Task<CreateOrderResponse> Handle(CreateOrderRequest request, CancellationToken ct)
    {
        var total = request.Items.Sum(i => i.Quantity * await _repository.GetPriceAsync(i.ProductId, ct));
        var paymentResult = await _payment.AuthorizeAsync(request.UserId, total, ct);
        
        if (!paymentResult.Success)
            throw new PaymentException(paymentResult.Reason);

        var order = new Order
        {
            UserId = req

uest.UserId, Items = request.Items, Address = request.Address, CreatedAt = DateTimeOffset.UtcNow };

    await _repository.SaveAsync(order, ct);
    return new CreateOrderResponse(order.Id, order.CreatedAt);
}

}

**4. Add Validation Boundary**
FluentValidation enforces constraints before handlers execute.

```csharp
public class CreateOrderValidator : AbstractValidator<CreateOrderRequest>
{
    public CreateOrderValidator()
    {
        RuleFor(x => x.UserId).NotEmpty();
        RuleFor(x => x.Items).NotEmpty().Must(items => items.All(i => i.Quantity > 0));
        RuleFor(x => x.Address).NotNull().SetValidator(new ShippingAddressValidator());
    }
}

5. Map Endpoint to Handler Minimal APIs route directly to MediatR. No controllers required.

app.MapPost("/api/orders", async (
    CreateOrderRequest request, 
    IMediator mediator, 
    CancellationToken ct) =>
{
    var result = await mediator.Send(request, ct);
    return Results.Created($"/api/orders/{result.OrderId}", result);
})
.WithName("CreateOrder")
.Produces<CreateOrderResponse>(StatusCodes.Status201Created)
.ProducesProblem(StatusCodes.Status400BadRequest);

6. Register Cross-Cutting Pipeline Validation, logging, and error handling are injected as behaviors.

builder.Services.AddMediatR(cfg => 
    cfg.RegisterServicesFromAssembly(typeof(CreateOrderHandler).Assembly));

builder.Services.AddTransient(
    typeof(IPipelineBehavior<,>), 
    typeof(ValidationBehavior<,>));

builder.Services.AddTransient(
    typeof(IPipelineBehavior<,>), 
    typeof(ExceptionHandlingBehavior<,>));

Architecture Decisions and Rationale

• Vertical Slices over Layers: Layered architectures force changes to ripple across controllers, services, and repositories. Vertical slices isolate use cases, reducing merge conflicts and dependency churn.
• MediatR for Routing: Decouples HTTP transport from business logic. Handlers remain framework-agnostic, enabling reuse in background workers, CLI tools, or gRPC services.
• Pipeline Behaviors over Middleware: Middleware operates at the HTTP pipeline level, making it difficult to scope validation or logging to specific endpoints. Pipeline behaviors attach to requests, providing explicit, testable boundaries.
• Explicit Contracts: Records enforce immutability and structural equality. They integrate cleanly with OpenAPI generation, client SDKs, and static analyzers.

Pitfall Guide

1. Fat Endpoints

Embedding business logic, data access, and validation directly in MapPost or controller methods creates untestable code. Fix: Extract logic into handlers. Endpoints should only route, map, and return HTTP results.

2. Over-Abstraction

Creating IUserService, UserServiceBase, UserServiceDecorator, and UserServiceFactory for simple CRUD operations adds cognitive overhead without measurable benefit. Fix: Apply abstractions only when multiple implementations exist or when cross-cutting concerns require interception.

3. Silent Validation Failures

Relying on ModelState.IsValid without mapping errors to a consistent response structure leaves clients guessing. Fix: Use FluentValidation with a pipeline behavior that transforms ValidationResult into a standardized ProblemDetails payload.

4. Mixed Concerns in Handlers

Injecting ILogger, IHttpContextAccessor, or IConfiguration into handlers couples business logic to infrastructure. Fix: Keep handlers pure. Move logging, configuration reads, and HTTP context access to pipeline behaviors or endpoint middleware.

5. Inconsistent Error Modeling

Returning raw exceptions, strings, or custom objects across endpoints breaks client SDK generation and monitoring pipelines. Fix: Enforce a single error contract (ProblemDetails or custom ApiError) via a global exception handler pipeline behavior.

6. N+1 Query Patterns

Loading related entities inside loops or without eager loading causes database round-trip explosions. Fix: Use repository methods that project directly to DTOs, or leverage EF Core Include/Select at the query boundary. Profile queries early in development.

7. Versioning Without Backward Compatibility

Breaking changes in v2 without deprecation windows cause client outages. Fix: Implement URL or header-based versioning with explicit compatibility matrices. Mark obsolete endpoints with ObsoleteAttribute and monitor usage via telemetry before removal.

Production Bundle

Action Checklist

• Contract-first design: Define request/response records before writing handlers
• Vertical slice organization: Group code by use case, not technical layer
• Pipeline behavior injection: Register validation, logging, and error handling as behaviors
• Explicit error mapping: Standardize all failures to ProblemDetails or ApiError
• Query boundary enforcement: Prevent N+1 by projecting data at the repository level
• OpenAPI annotation: Add .WithName(), .Produces(), and .ProducesProblem() to every endpoint
• Telemetry integration: Attach correlation IDs and business metrics to pipeline behaviors
• Deprecation policy: Version endpoints with explicit sunset timelines and usage tracking

Decision Matrix

Scenario	Recommended Approach	Why	Cost Impact
High-frequency CRUD operations	Minimal API + Repository	Low overhead, fast implementation, predictable performance	Low initial, medium maintenance
Complex business workflows	CQRS + MediatR	Separates read/write models, enables event sourcing, scales independently	High initial, low long-term
Multi-tenant SaaS platform	Vertical Slice + Policy-Based Auth	Isolates tenant context, reduces cross-tenant leakage risk	Medium initial, high reliability
Legacy migration	Anti-Corruption Layer + Facade	Protects new code from legacy data models, enables incremental rollout	High initial, reduces migration risk
Real-time event processing	MediatR + Background Workers	Reuses domain handlers, avoids duplication across HTTP and queue pipelines	Low incremental, high consistency

Configuration Template

// Program.cs
var builder = WebApplication.CreateBuilder(args);

builder.Services.AddMediatR(cfg => 
    cfg.RegisterServicesFromAssembly(typeof(Program).Assembly));

builder.Services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));
builder.Services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ExceptionHandlingBehavior<,>));
builder.Services.AddTransient(typeof(IPipelineBehavior<,>), typeof(TelemetryBehavior<,>));

builder.Services.AddProblemDetails();
builder.Services.AddOpenApi();

var app = builder.Build();

app.UseExceptionHandler();
app.UseStatusCodePages();
app.MapOpenApi();
app.MapControllers(); // Only if legacy controllers exist

app.MapGroup("/api/v1")
   .WithOpenApi()
   .MapCreateOrder()
   .MapGetOrder();

app.Run();

// appsettings.json
{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",
  "ApiBehavior": {
    "SuppressModelStateInvalidFilter": true,
    "InvalidModelStateResponseFactory": "ProblemDetails"
  },
  "Telemetry": {
    "EnableCorrelationId": true,
    "EnableBusinessMetrics": true
  }
}

Quick Start Guide

1. Install packages: dotnet add package MediatR, dotnet add package FluentValidation, dotnet add package FluentValidation.AspNetCore
2. Create a Features/ directory and add a CreateOrder.cs file containing the request record, validator, and handler
3. Register MediatR and pipeline behaviors in Program.cs, then map the endpoint using app.MapPost()
4. Run dotnet run and verify OpenAPI output at /swagger/index.html
5. Send a test request with curl -X POST http://localhost:5000/api/v1/orders -H "Content-Type: application/json" -d '{"userId":"...","items":[],"address":{}}' and observe validation/error handling in action