Minimal APIs

Trigger On

• building new HTTP APIs in ASP.NET Core
• creating lightweight microservices
• choosing between Minimal APIs and controllers
• organizing endpoints with route groups
• implementing validation and filters

Documentation

• Minimal APIs Overview
• Minimal API Tutorial
• Filters in Minimal APIs
• OpenAPI Support
• Route Groups

References

• patterns.md - detailed route groups, filters, TypedResults patterns, parameter binding, error handling, and testing
• anti-patterns.md - common Minimal API mistakes to avoid

When to Use Minimal APIs vs Controllers

Workflow

1. Define endpoints directly in Program.cs (for small APIs)
2. Use route groups for related endpoints
3. Move handlers to separate classes as the API grows
4. Apply filters for cross-cutting concerns
5. Use TypedResults for type-safe responses
6. Generate OpenAPI docs with .WithOpenApi()

Basic Patterns

Simple Endpoints

var app = builder.Build();

app.MapGet("/", () => "Hello World");

app.MapGet("/products/{id}", (int id) => Results.Ok(new { Id = id }));

app.MapPost("/products", (Product product) => Results.Created($"/products/{product.Id}", product));

TypedResults (Strongly-Typed)

app.MapGet("/products/{id}", Results<Ok<Product>, NotFound> (int id, AppDb db) =>
{
    var product = db.Products.Find(id);
    return product is not null
        ? TypedResults.Ok(product)
        : TypedResults.NotFound();
});

Dependency Injection

app.MapGet("/products", async (IProductService service) =>
{
    return await service.GetAllAsync();
});

// Or with [FromServices] for clarity
app.MapGet("/products", async ([FromServices] IProductService service) =>
    await service.GetAllAsync());

Route Groups

Basic Grouping

var products = app.MapGroup("/api/products");

products.MapGet("/", GetAll);
products.MapGet("/{id}", GetById);
products.MapPost("/", Create);
products.MapPut("/{id}", Update);
products.MapDelete("/{id}", Delete);

Groups with Shared Configuration

var api = app.MapGroup("/api")
    .RequireAuthorization()
    .AddEndpointFilter<ValidationFilter>();

var products = api.MapGroup("/products")
    .WithTags("Products");

var orders = api.MapGroup("/orders")
    .WithTags("Orders")
    .RequireAuthorization("AdminOnly");

Endpoint Filters

Inline Filter

app.MapGet("/products/{id}", (int id) => Results.Ok(id))
    .AddEndpointFilter(async (context, next) =>
    {
        var id = context.GetArgument<int>(0);
        if (id <= 0)
            return Results.BadRequest("Invalid ID");

        return await next(context);
    });

Class-Based Filter

public class ValidationFilter<T> : IEndpointFilter where T : class
{
    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        var argument = context.Arguments
            .OfType<T>()
            .FirstOrDefault();

        if (argument is null)
            return Results.BadRequest("Invalid request body");

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

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

        return await next(context);
    }
}

// Usage
products.MapPost("/", Create)
    .AddEndpointFilter<ValidationFilter<CreateProductRequest>>();

Global Filters via Root Group

// All endpoints inherit filters from root group
var root = app.MapGroup("")
    .AddEndpointFilter<LoggingFilter>()
    .AddEndpointFilter<ErrorHandlingFilter>();

root.MapGet("/health", () => Results.Ok());
root.MapGroup("/api/products").MapGet("/", GetProducts);

Organizing Larger APIs

Extension Method Pattern

// ProductEndpoints.cs
public static class ProductEndpoints
{
    public static RouteGroupBuilder MapProductEndpoints(this IEndpointRouteBuilder app)
    {
        var group = app.MapGroup("/api/products")
            .WithTags("Products");

        group.MapGet("/", GetAll);
        group.MapGet("/{id}", GetById);
        group.MapPost("/", Create);

        return group;
    }

    private static async Task<Ok<List<Product>>> GetAll(IProductService service)
        => TypedResults.Ok(await service.GetAllAsync());

    private static async Task<Results<Ok<Product>, NotFound>> GetById(
        int id, IProductService service)
    {
        var product = await service.GetByIdAsync(id);
        return product is not null
            ? TypedResults.Ok(product)
            : TypedResults.NotFound();
    }

    private static async Task<Created<Product>> Create(
        CreateProductRequest request, IProductService service)
    {
        var product = await service.CreateAsync(request);
        return TypedResults.Created($"/api/products/{product.Id}", product);
    }
}

// Program.cs
app.MapProductEndpoints();
app.MapOrderEndpoints();

Request/Response DTOs

// Separate from domain models
public record CreateProductRequest(string Name, decimal Price);
public record UpdateProductRequest(string Name, decimal Price);
public record ProductResponse(int Id, string Name, decimal Price);

// Don't expose domain entities directly
app.MapPost("/products", (CreateProductRequest request, IMapper mapper) =>
{
    var product = mapper.Map<Product>(request);
    // ...
    return TypedResults.Created($"/products/{product.Id}",
        mapper.Map<ProductResponse>(product));
});

Anti-Patterns to Avoid

OpenAPI Integration

builder.Services.AddOpenApi();

app.MapOpenApi();  // Serves OpenAPI spec

app.MapGet("/products", GetProducts)
    .WithName("GetProducts")
    .WithSummary("Get all products")
    .WithDescription("Returns a list of all available products")
    .Produces<List<Product>>(StatusCodes.Status200OK)
    .ProducesProblem(StatusCodes.Status500InternalServerError);

Deliver

• clean, organized Minimal API endpoints
• proper use of route groups and filters
• type-safe responses with TypedResults
• OpenAPI documentation
• validation with endpoint filters

Validate

• endpoints return correct status codes
• validation filters catch invalid input
• OpenAPI spec is accurate
• route groups share common configuration
• handlers are testable (can mock dependencies)