Minimal API

// Extensions/IEndpointGroup.cs
public interface IEndpointGroup
{
    void Map(IEndpointRouteBuilder app);
}

// Extensions/EndpointExtensions.cs
public static class EndpointExtensions
{
    public static WebApplication MapEndpoints(this WebApplication app)
    {
        var groups = typeof(Program).Assembly
            .GetTypes()
            .Where(t => t.IsAssignableTo(typeof(IEndpointGroup)) && !t.IsInterface && !t.IsAbstract)
            .Select(Activator.CreateInstance)
            .Cast<IEndpointGroup>();

        foreach (var group in groups)
            group.Map(app);

        return app;
    }
}

// Program.cs — this NEVER changes when adding endpoints
var app = builder.Build();
app.MapEndpoints();
app.Run();

// Features/Orders/OrderEndpoints.cs — one file per endpoint group
public sealed class OrderEndpoints : IEndpointGroup
{
    public void Map(IEndpointRouteBuilder app)
    {
        var group = app.MapGroup("/api/orders").WithTags("Orders");

        group.MapPost("/", CreateOrder)
            .WithName("CreateOrder")
            .WithSummary("Create a new order")
            .Produces<OrderResponse>(StatusCodes.Status201Created)
            .ProducesValidationProblem()
            .RequireAuthorization();

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

        group.MapGet("/", ListOrders)
            .WithName("ListOrders")
            .Produces<PagedList<OrderResponse>>();
    }

    private static async Task<Results<Created<OrderResponse>, ValidationProblem>> CreateOrder(
        CreateOrderRequest request,
        ISender sender,
        CancellationToken ct)
    {
        var result = await sender.Send(new CreateOrder.Command(request.CustomerId, request.Items), ct);
        return result.IsSuccess
            ? TypedResults.Created($"/api/orders/{result.Value.Id}", result.Value)
            : TypedResults.ValidationProblem(result.Errors);
    }

    private static async Task<Results<Ok<OrderResponse>, NotFound>> GetOrder(
        Guid id,
        ISender sender,
        CancellationToken ct)
    {
        var result = await sender.Send(new GetOrder.Query(id), ct);
        return result.IsSuccess
            ? TypedResults.Ok(result.Value)
            : TypedResults.NotFound();
    }

    private static async Task<Ok<PagedList<OrderResponse>>> ListOrders(
        [AsParameters] ListOrdersQuery query,
        ISender sender,
        CancellationToken ct)
    {
        var result = await sender.Send(query, ct);
        return TypedResults.Ok(result);
    }
}

// GOOD — TypedResults with union return type
private static async Task<Results<Ok<Product>, NotFound, ValidationProblem>> GetProduct(
    Guid id,
    AppDbContext db,
    CancellationToken ct)
{
    var product = await db.Products.FindAsync([id], ct);
    return product is not null
        ? TypedResults.Ok(product)
        : TypedResults.NotFound();
}

// Route parameters
app.MapGet("/orders/{id:guid}", (Guid id) => ...);

// Query parameters (nullable = optional)
app.MapGet("/orders", (int page, int? pageSize, string? status) => ...);

// Complex query parameters with [AsParameters]
public record ListOrdersQuery(int Page = 1, int PageSize = 20, string? Status = null);
app.MapGet("/orders", ([AsParameters] ListOrdersQuery query) => ...);

// Header binding
app.MapGet("/orders", ([FromHeader(Name = "X-Correlation-Id")] string? correlationId) => ...);

// DI services are auto-resolved (no attribute needed)
app.MapPost("/orders", (CreateOrderRequest request, ISender sender) => ...);

// Apply the canonical filter (see error-handling skill) to a mutating endpoint
group.MapPost("/", CreateOrder)
    .AddEndpointFilter<ValidationFilter<CreateOrderRequest>>();

// Apply a filter to a group (affects all endpoints in the group)
group.AddEndpointFilter<LoggingFilter>();

// Program.cs — service registration only, no endpoint wiring
builder.Services.AddOpenApi();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
}
app.MapEndpoints(); // auto-discovers all IEndpointGroup implementations

// Endpoint metadata enriches the OpenAPI spec
group.MapPost("/", CreateOrder)
    .WithName("CreateOrder")
    .WithSummary("Create a new order")
    .WithDescription("Creates a new order for the specified customer with the given line items.")
    .Produces<OrderResponse>(StatusCodes.Status201Created)
    .ProducesValidationProblem()
    .ProducesProblem(StatusCodes.Status500InternalServerError);

builder.Services.AddRateLimiter(options =>
{
    options.AddFixedWindowLimiter("api", opt =>
    {
        opt.PermitLimit = 100;
        opt.Window = TimeSpan.FromMinutes(1);
    });
});

// Apply inside an IEndpointGroup.Map method
var group = app.MapGroup("/api/orders")
    .WithTags("Orders")
    .RequireRateLimiting("api");

builder.Services.AddOutputCache(options =>
{
    options.AddBasePolicy(builder => builder.Expire(TimeSpan.FromMinutes(5)));
    options.AddPolicy("ByIdCache", builder => builder
        .Expire(TimeSpan.FromMinutes(10))
        .SetVaryByRouteValue("id"));
});

group.MapGet("/{id:guid}", GetOrder)
    .CacheOutput("ByIdCache");

// BAD — endpoints scattered in Program.cs
app.MapGet("/orders", async (AppDbContext db) => await db.Orders.ToListAsync());
app.MapGet("/orders/{id}", async (Guid id, AppDbContext db) => await db.Orders.FindAsync(id));
app.MapPost("/orders", async (Order order, AppDbContext db) => { /* ... */ });
app.MapGet("/products", async (AppDbContext db) => await db.Products.ToListAsync());

// ALSO BAD — manual MapGroup calls in Program.cs (grows with every feature)
app.MapGroup("/api/orders").WithTags("Orders").MapOrderEndpoints();
app.MapGroup("/api/products").WithTags("Products").MapProductEndpoints();
app.MapGroup("/api/customers").WithTags("Customers").MapCustomerEndpoints();
// Program.cs grows every time you add a feature...

// GOOD — auto-discovered, Program.cs never changes
app.MapEndpoints(); // discovers all IEndpointGroup implementations

// BAD — Results.Ok doesn't contribute to OpenAPI schema
private static async Task<IResult> GetOrder(Guid id, AppDbContext db)
{
    var order = await db.Orders.FindAsync(id);
    return order is not null ? Results.Ok(order) : Results.NotFound();
}

// GOOD — TypedResults with explicit union type
private static async Task<Results<Ok<Order>, NotFound>> GetOrder(Guid id, AppDbContext db)
{
    var order = await db.Orders.FindAsync(id);
    return order is not null ? TypedResults.Ok(order) : TypedResults.NotFound();
}

// BAD — leaks internal structure, can't evolve independently
app.MapGet("/orders/{id}", async (Guid id, AppDbContext db) =>
    await db.Orders.Include(o => o.Items).FirstOrDefaultAsync(o => o.Id == id));

// GOOD — map to a response DTO
app.MapGet("/orders/{id}", async (Guid id, AppDbContext db) =>
{
    var order = await db.Orders
        .Where(o => o.Id == id)
        .Select(o => new OrderResponse(o.Id, o.Total, o.CreatedAt))
        .FirstOrDefaultAsync();
    return order is not null ? TypedResults.Ok(order) : TypedResults.NotFound();
});