dotnet-architecture-patterns

Modern architecture patterns for .NET applications. Covers practical approaches to organizing minimal APIs at scale, vertical slice architecture, request pipeline composition, validation strategies, caching, error handling, and idempotency/outbox patterns.

Out of scope: DI container mechanics and async/await patterns -- see [skill:dotnet-csharp-dependency-injection] and [skill:dotnet-csharp-async-patterns]. Project scaffolding and file layout -- see [skill:dotnet-scaffold-project]. Testing strategies -- see [skill:dotnet-testing-strategy] for decision guidance and [skill:dotnet-integration-testing] for WebApplicationFactory patterns.

Cross-references: [skill:dotnet-csharp-dependency-injection] for service registration and lifetimes, [skill:dotnet-csharp-async-patterns] for async pipeline patterns, [skill:dotnet-csharp-configuration] for Options pattern in configuration, [skill:dotnet-solid-principles] for SOLID/DRY design principles governing class and interface design.

---

Vertical Slice Architecture

Organize code by feature (vertical slice) rather than by technical layer (controllers, services, repositories). Each slice owns its endpoint, handler, validation, and data access.

Directory Structure

Features/
  Orders/
    CreateOrder/
      CreateOrderEndpoint.cs
      CreateOrderHandler.cs
      CreateOrderRequest.cs
      CreateOrderValidator.cs
    GetOrder/
      GetOrderEndpoint.cs
      GetOrderHandler.cs
    ListOrders/
      ListOrdersEndpoint.cs
      ListOrdersHandler.cs
  Products/
    GetProduct/
      ...

Why Vertical Slices

• Low coupling: changing one feature does not ripple through shared layers
• Easy navigation: everything for a feature is in one place
• Independent testability: each slice has a clear input/output contract
• Team scalability: different developers can work on different features without merge conflicts

Slice Anatomy

Each slice typically contains:

1. Request/Response DTOs -- the contract
2. Validator -- input validation rules
3. Handler -- business logic
4. Endpoint -- HTTP mapping (route, method, status codes)

// Features/Orders/CreateOrder/CreateOrderRequest.cs
public sealed record CreateOrderRequest(
    string CustomerId,
    List<OrderLineRequest> Lines);

public sealed record OrderLineRequest(
    string ProductId,
    int Quantity);

// Features/Orders/CreateOrder/CreateOrderResponse.cs
public sealed record CreateOrderResponse(
    string OrderId,
    decimal Total,
    DateTimeOffset CreatedAt);

---

Minimal API Organization at Scale

Route Group Pattern

Use MapGroup to organize related endpoints and apply shared filters:

// Program.cs
var app = builder.Build();

app.MapGroup("/api/orders")
   .WithTags("Orders")
   .MapOrderEndpoints();

app.MapGroup("/api/products")
   .WithTags("Products")
   .MapProductEndpoints();

app.Run();

// Features/Orders/OrderEndpoints.cs
public static class OrderEndpoints
{
    public static RouteGroupBuilder MapOrderEndpoints(this RouteGroupBuilder group)
    {
        group.MapPost("/", CreateOrderEndpoint.Handle)
             .WithName("CreateOrder")
             .Produces<CreateOrderResponse>(StatusCodes.Status201Created)
             .ProducesValidationProblem();

        group.MapGet("/{id}", GetOrderEndpoint.Handle)
             .WithName("GetOrder")
             .Produces<OrderResponse>()
             .ProducesProblem(StatusCodes.Status404NotFound);

        group.MapGet("/", ListOrdersEndpoint.Handle)
             .WithName("ListOrders")
             .Produces<PagedResult<OrderSummary>>();

        return group;
    }
}

Endpoint Classes

Keep each endpoint in its own static class with a single Handle method:

public static class CreateOrderEndpoint
{
    public static async Task<IResult> Handle(
        CreateOrderRequest request,
        IValidator<CreateOrderRequest> validator,
        IOrderService orderService,
        CancellationToken ct)
    {
        var validation = await validator.ValidateAsync(request, ct);
        if (!validation.IsValid)
        {
            return Results.ValidationProblem(validation.ToDictionary());
        }

        var order = await orderService.CreateAsync(request, ct);

        return Results.Created($"/api/orders/{order.OrderId}", order);
    }
}

---

Request Pipeline Composition

Endpoint Filters (Middleware for Endpoints)

Use endpoint filters for cross-cutting concerns scoped to specific routes:

// Validation filter applied to a route group
public sealed class ValidationFilter<TRequest> : IEndpointFilter
    where TRequest : class
{
    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        var request = context.Arguments.OfType<TRequest>().FirstOrDefault();
        if (request is null)
        {
            return Results.BadRequest();
        }

        var validator = context.HttpContext.RequestServices
            .GetService<IValidator<TRequest>>();

        if (validator is not null)
        {
            var result = await validator.ValidateAsync(request);
            if (!result.IsValid)
            {
                return Results.ValidationProblem(result.ToDictionary());
            }
        }

        return await next(context);
    }
}

// Usage
group.MapPost("/", CreateOrderEndpoint.Handle)
     .AddEndpointFilter<ValidationFilter<CreateOrderRequest>>();

Pipeline Order

The standard middleware pipeline order matters:

app.UseExceptionHandler();       // 1. Global error handling
app.UseStatusCodePages();        // 2. Status code formatting
app.UseRateLimiter();            // 3. Rate limiting
app.UseAuthentication();         // 4. Authentication
app.UseAuthorization();          // 5. Authorization
// Endpoint routing happens here

---

Error Handling

Problem Details (RFC 9457)

Use the built-in Problem Details support for consistent error responses:

// Program.cs
builder.Services.AddProblemDetails(options =>
{
    options.CustomizeProblemDetails = context =>
    {
        context.ProblemDetails.Extensions["traceId"] =
            context.HttpContext.TraceIdentifier;
    };
});

app.UseExceptionHandler();
app.UseStatusCodePages();

Result Pattern for Business Logic

Return a result type from handlers instead of throwing exceptions for expected business failures:

public abstract record Result<T>
{
    public sealed record Success(T Value) : Result<T>;
    public sealed record NotFound(string Message) : Result<T>;
    public sealed record ValidationFailed(IDictionary<string, string[]> Errors) : Result<T>;
    public sealed record Conflict(string Message) : Result<T>;
}

// In the handler
public async Task<Result<Order>> CreateAsync(
    CreateOrderRequest request,
    CancellationToken ct)
{
    var customer = await _db.Customers.FindAsync([request.CustomerId], ct);
    if (customer is null)
    {
        return new Result<Order>.NotFound($"Customer {request.CustomerId} not found");
    }

    // ... create order
    return new Result<Order>.Success(order);
}

// In the endpoint -- map result to HTTP response
return result switch
{
    Result<Order>.Success s => Results.Created($"/api/orders/{s.Value.Id}", s.Value),
    Result<Order>.NotFound n => Results.Problem(n.Message, statusCode: 404),
    Result<Order>.ValidationFailed v => Results.ValidationProblem(v.Errors),
    Result<Order>.Conflict c => Results.Problem(c.Message, statusCode: 409),
    _ => Results.Problem("Unexpected error", statusCode: 500)
};

---

Validation Strategy

Choose validation based on complexity. Prefer built-in mechanisms as the default; reserve FluentValidation for complex business rules that outgrow declarative attributes. For detailed framework guidance, see [skill:dotnet-input-validation]. For SOLID principles governing where validation belongs in your architecture, see [skill:dotnet-solid-principles].

Built-in: Data Annotations + MiniValidation (Default)

Start with Data Annotations for simple property-level constraints. Use MiniValidation for lightweight validation in Minimal APIs without MVC model binding overhead. For .NET 10+ projects, prefer the built-in AddValidation() source-generator pipeline (see [skill:dotnet-input-validation]).

public sealed record CreateProductRequest(
    [Required, MaxLength(200)] string Name,
    [Range(0.01, double.MaxValue)] decimal Price);

// In endpoint
if (!MiniValidator.TryValidate(request, out var errors))
{
    return Results.ValidationProblem(errors);
}

FluentValidation (Opt-in for Complex Rules)

When validation rules outgrow annotations -- cross-property rules, conditional logic, database-dependent checks -- use FluentValidation. Register via assembly scanning and apply through endpoint filters or manual validation:

// Register validators by assembly scanning
builder.Services.AddValidatorsFromAssemblyContaining<Program>(ServiceLifetime.Scoped);

// Validator implementation
public sealed class CreateOrderValidator : AbstractValidator<CreateOrderRequest>
{
    public CreateOrderValidator()
    {
        RuleFor(x => x.CustomerId)
            .NotEmpty()
            .MaximumLength(50);

        RuleFor(x => x.Lines)
            .NotEmpty()
            .WithMessage("Order must have at least one line item");

        RuleForEach(x => x.Lines)
            .ChildRules(line =>
            {
                line.RuleFor(l => l.ProductId).NotEmpty();
                line.RuleFor(l => l.Quantity).GreaterThan(0);
            });
    }
}

---

Caching Strategy

Output Caching (HTTP Response Caching)

builder.Services.AddOutputCache(options =>
{
    options.AddBasePolicy(p => p.NoCache());

    options.AddPolicy("ProductList", p =>
        p.Expire(TimeSpan.FromMinutes(5))
         .Tag("products"));

    options.AddPolicy("ProductDetail", p =>
        p.Expire(TimeSpan.FromMinutes(10))
         .SetVaryByRouteValue("id")
         .Tag("products"));
});

app.UseOutputCache();

// Apply to endpoints
group.MapGet("/", ListProductsEndpoint.Handle)
     .CacheOutput("ProductList");

group.MapGet("/{id}", GetProductEndpoint.Handle)
     .CacheOutput("ProductDetail");

// Invalidate by tag
app.MapPost("/api/products", async (
    IOutputCacheStore cache,
    /* ... */) =>
{
    // ... create product
    await cache.EvictByTagAsync("products", ct);
    return Results.Created(/* ... */);
});

Distributed Caching (Application-Level)

builder.Services.AddStackExchangeRedisCache(options =>
{
    options.Configuration = builder.Configuration
        .GetConnectionString("Redis");
});

// Usage with IDistributedCache
public sealed class ProductService(
    IDistributedCache cache,
    AppDbContext db)
{
    public async Task<Product?> GetByIdAsync(
        string id, CancellationToken ct = default)
    {
        var cacheKey = $"product:{id}";
        var cached = await cache.GetStringAsync(cacheKey, ct);

        if (cached is not null)
        {
            return JsonSerializer.Deserialize<Product>(cached);
        }

        var product = await db.Products.FindAsync([id], ct);
        if (product is not null)
        {
            await cache.SetStringAsync(
                cacheKey,
                JsonSerializer.Serialize(product),
                new DistributedCacheEntryOptions
                {
                    AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(10)
                },
                ct);
        }

        return product;
    }
}

HybridCache (.NET 9+)

HybridCache combines L1 (in-memory) and L2 (distributed) caching with stampede protection:

builder.Services.AddHybridCache(options =>
{
    options.DefaultEntryOptions = new HybridCacheEntryOptions
    {
        Expiration = TimeSpan.FromMinutes(10),
        LocalCacheExpiration = TimeSpan.FromMinutes(2)
    };
});

// Usage -- stampede-safe, two-tier
public sealed class ProductService(HybridCache cache, AppDbContext db)
{
    public async Task<Product?> GetByIdAsync(
        string id, CancellationToken ct = default)
    {
        return await cache.GetOrCreateAsync(
            $"product:{id}",
            async cancel => await db.Products.FindAsync([id], cancel),
            cancellationToken: ct);
    }
}

---

Idempotency and Outbox Pattern

Idempotency Keys

Prevent duplicate processing of retried requests. A robust idempotency implementation must:

1. Scope keys by route + user/tenant to prevent cross-endpoint collisions
2. Atomically claim the key before executing, so concurrent duplicates are rejected
3. Store a concrete response envelope (not an IResult reference) for safe replay

Database-Backed Idempotency (Recommended)

Use a database row with a unique constraint for atomic claim-then-execute:

// Idempotency record stored alongside domain data
public sealed class IdempotencyRecord
{
    public required string Key { get; init; }         // Scoped key
    public required string RequestRoute { get; init; }
    public required string? UserId { get; init; }
    public int StatusCode { get; set; }
    public string? ResponseBody { get; set; }         // Serialized JSON
    public string? ContentType { get; set; }
    public DateTimeOffset CreatedAt { get; init; } = DateTimeOffset.UtcNow;
    public bool IsCompleted { get; set; }
}

public sealed class IdempotencyFilter(AppDbContext db) : IEndpointFilter
{
    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        var httpContext = context.HttpContext;
        if (!httpContext.Request.Headers.TryGetValue(
            "Idempotency-Key", out var keyValues))
        {
            return await next(context);
        }

        var clientKey = keyValues.ToString();
        if (string.IsNullOrWhiteSpace(clientKey) || clientKey.Length > 256)
        {
            return Results.Problem("Invalid Idempotency-Key", statusCode: 400);
        }

        // Scope key by route + user to prevent cross-endpoint/cross-tenant collisions
        var route = $"{httpContext.Request.Method}:{httpContext.Request.Path}";
        var userId = httpContext.User.FindFirst("sub")?.Value ?? "anonymous";
        var scopedKey = $"{route}:{userId}:{clientKey}";

        // Check for existing record (completed = replay, in-progress = reject)
        var existing = await db.IdempotencyRecords
            .FirstOrDefaultAsync(r => r.Key == scopedKey);

        if (existing is { IsCompleted: true })
        {
            // Replay: value responses get body, non-value responses get status only
            if (existing.ResponseBody is not null)
            {
                return Results.Text(
                    existing.ResponseBody,
                    existing.ContentType ?? "application/json",
                    statusCode: existing.StatusCode);
            }

            return Results.StatusCode(existing.StatusCode);
        }

        if (existing is { IsCompleted: false })
        {
            // Another request claimed this key but hasn't completed yet.
            // Reject to prevent duplicate execution.
            return Results.Problem(
                "Duplicate request in progress", statusCode: 409);
        }

        // Atomic claim: insert with unique constraint -- concurrent duplicate
        // requests will throw DbUpdateException and get a 409 Conflict
        {
            var record = new IdempotencyRecord
            {
                Key = scopedKey,
                RequestRoute = route,
                UserId = userId,
                IsCompleted = false
            };
            db.IdempotencyRecords.Add(record);

            try
            {
                await db.SaveChangesAsync();
            }
            catch (DbUpdateException)
            {
                return Results.Problem(
                    "Duplicate request in progress", statusCode: 409);
            }

            existing = record;
        }

        // Execute the actual handler
        var result = await next(context);

        // Always finalize the record -- handles both value and non-value results
        // (Results.Ok(obj), Results.NoContent(), Results.Accepted(), etc.)
        existing.StatusCode = result is IStatusCodeHttpResult statusResult
            ? statusResult.StatusCode ?? 200
            : 200;
        existing.ResponseBody = result is IValueHttpResult valueResult
            ? JsonSerializer.Serialize(valueResult.Value)
            : null;  // No body for non-value results (204, 202, etc.)
        existing.ContentType = existing.ResponseBody is not null
            ? "application/json"
            : null;
        existing.IsCompleted = true;
        await db.SaveChangesAsync();

        return result;
    }
}

Key design choices:

• Three states: no record (claim it), in-progress (reject 409), completed (replay cached response)
• Unique constraint on Key column provides atomic claim without distributed locks
• Scoped key (route:userId:clientKey) prevents cross-endpoint and cross-tenant collisions
• Response envelope stores serialized body + status code + content type (not IResult references)
• In-progress records (claimed but not completed) return 409 to concurrent duplicates
• Consider adding a stale-record cleanup job to handle abandoned in-progress records (e.g., process crashed mid-execution)

Transactional Outbox Pattern

Guarantee at-least-once delivery of domain events alongside database writes:

// 1. Store outbox messages in the same transaction as the domain write
public sealed class OutboxMessage
{
    public Guid Id { get; init; } = Guid.NewGuid();
    public required string EventType { get; init; }
    public required string Payload { get; init; }
    public DateTimeOffset CreatedAt { get; init; } = DateTimeOffset.UtcNow;
    public DateTimeOffset? ProcessedAt { get; set; }
}

// 2. In the handler -- same DbContext transaction
public async Task<Order> CreateOrderAsync(
    CreateOrderRequest request,
    CancellationToken ct)
{
    await using var transaction = await _db.Database
        .BeginTransactionAsync(ct);

    var order = new Order { /* ... */ };
    _db.Orders.Add(order);

    _db.OutboxMessages.Add(new OutboxMessage
    {
        EventType = "OrderCreated",
        Payload = JsonSerializer.Serialize(
            new OrderCreatedEvent(order.Id, order.Total))
    });

    await _db.SaveChangesAsync(ct);
    await transaction.CommitAsync(ct);

    return order;
}

// 3. Background processor publishes outbox messages
// See [skill:dotnet-background-services] for the Channels-based
// processor that polls and publishes these messages.

The outbox pattern ensures that if the database write succeeds, the event is guaranteed to be published (eventually), even if the message broker is temporarily unavailable.

---

Key Principles

• Apply SOLID principles -- Single Responsibility (one reason to change per class), Open/Closed (extend via new types, not modifying existing code), Dependency Inversion (depend on abstractions at module boundaries). See [skill:dotnet-solid-principles] for anti-patterns, fixes, and compliance tests.
• Prefer composition over inheritance -- use endpoint filters, middleware, and pipeline composition rather than base classes
• Keep slices independent -- avoid shared abstractions that couple features together; DRY applies to knowledge duplication, not code similarity across bounded contexts
• Validate early, fail fast -- validate at the boundary (endpoint filters) before entering business logic
• Use Problem Details everywhere -- consistent error format across all endpoints
• Cache at the right level -- output cache for HTTP responses, distributed cache for shared state, HybridCache for both
• Make writes idempotent -- use idempotency keys for any non-idempotent operation clients may retry

---

Agent Gotchas

1. Idempotency must handle three states -- An idempotency implementation must distinguish no-record (claim it), in-progress (reject duplicate), and completed (replay cached response). Check-then-act without guarding the in-progress state allows concurrent duplicate execution.
2. Always finalize idempotency records unconditionally -- Do NOT gate completion on specific IResult subtypes (e.g., IValueHttpResult). Non-value results like Results.NoContent() or Results.Accepted() would be left permanently stuck in the in-progress state.
3. Cache invalidation must be explicit -- When using output caching or distributed caching, ALWAYS invalidate (evict by tag or key) after write operations. Forgetting invalidation causes stale reads that are hard to debug.
4. HybridCache stampede protection only works with GetOrCreateAsync -- Do NOT use separate get-then-set patterns with HybridCache; use the factory overload so the library serializes concurrent requests for the same key.
5. Outbox messages must be written in the same transaction as domain data -- If you write the outbox message outside the domain transaction, a crash between the two writes loses the event. ALWAYS use BeginTransactionAsync to wrap both writes atomically.
6. Endpoint filter order matters -- Filters added first run outermost. A validation filter must run before an idempotency filter, otherwise invalid requests get cached as idempotent responses.
7. Do NOT share DbContext across concurrent requests -- DbContext is not thread-safe. Each request must resolve its own scoped instance from DI. Using a singleton or static DbContext causes data corruption under concurrency.

---

Knowledge Sources

Architecture patterns in this skill are grounded in publicly available content from:

• Jimmy Bogard's Vertical Slice Architecture -- Organizing code by feature instead of by technical layer. Bogard advocates that each vertical slice owns its own request, handler, validation, and data access, reducing cross-feature coupling. He originated the popular MediatR library for request/handler dispatch in .NET, though MediatR is now commercial for commercial use. When applying vertical slice guidance, prefer the built-in IEndpointFilter and handler pattern shown above rather than introducing a third-party mediator dependency for simple scenarios. Source: https://www.jimmybogard.com/vertical-slice-architecture/
• Jimmy Bogard's Domain-Driven Design Patterns -- Rich domain model guidance including entity design, value objects, domain events, and aggregate boundaries. Key insight: domain events should be dispatched after the aggregate state change is persisted (not before), to avoid inconsistency if persistence fails. Source: https://www.jimmybogard.com/
• Nick Chapsas' Modern .NET Patterns -- Practical patterns for modern .NET including result types for error handling, structured validation pipelines, and modern C# feature adoption in production codebases. Source: https://www.youtube.com/@nickchapsas

Note: This skill applies publicly documented guidance. It does not represent or speak for the named sources. MediatR is a commercial product for commercial use; the patterns here are demonstrated with built-in .NET mechanisms.

References

• ASP.NET Core Best Practices
• Minimal APIs overview
• Output caching middleware
• HybridCache library
• Problem Details (RFC 9457)
• Endpoint filters in minimal APIs
• Vertical Slice Architecture (Jimmy Bogard)