controller-patterns

name	controller-patterns
description	Use when building controller-based REST APIs with action results, model binding, or MediatR integration.
metadata	{"category":"api","agent":"api-designer"}
when_to_use	When creating or modifying controller-based API endpoints with MediatR integration

Controller Patterns

Core Principles

• Use [ApiController] for automatic model validation and binding behavior
• Follow RESTful route conventions with resource-based URLs
• Return ActionResult<T> with explicit [ProducesResponseType] attributes
• Delegate all logic to MediatR handlers — controllers are thin dispatchers
• Propagate CancellationToken from every action method

Patterns

Standard RESTful Controller

[ApiController]
[Route("api/[controller]")]
[Produces("application/json")]
public sealed class OrdersController(ISender sender) : ControllerBase
{
    [HttpGet]
    [ProducesResponseType(typeof(PagedList<OrderResponse>),
        StatusCodes.Status200OK)]
    public async Task<ActionResult<PagedList<OrderResponse>>> GetOrders(
        [FromQuery] OrderFilter filter, CancellationToken ct)
    {
        var result = await sender.Send(new ListOrdersQuery(filter), ct);
        return Ok(result);
    }

    [HttpGet("{id:guid}")]
    [ProducesResponseType(typeof(OrderResponse), StatusCodes.Status200OK)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public async Task<ActionResult<OrderResponse>> GetOrder(
        Guid id, CancellationToken ct)
    {
        var result = await sender.Send(new GetOrderQuery(id), ct);
        return result is not null ? Ok(result) : NotFound();
    }

    [HttpPost]
    [ProducesResponseType(typeof(OrderResponse),
        StatusCodes.Status201Created)]
    [ProducesResponseType(typeof(ProblemDetails),
        StatusCodes.Status400BadRequest)]
    public async Task<ActionResult<OrderResponse>> CreateOrder(
        CreateOrderRequest request, CancellationToken ct)
    {
        var result = await sender.Send(
            new CreateOrderCommand(request.CustomerName, request.Items), ct);
        return CreatedAtAction(
            nameof(GetOrder), new { id = result.Id }, result);
    }

    [HttpPut("{id:guid}")]
    [ProducesResponseType(StatusCodes.Status204NoContent)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public async Task<IActionResult> UpdateOrder(
        Guid id, UpdateOrderRequest request, CancellationToken ct)
    {
        var result = await sender.Send(
            new UpdateOrderCommand(id, request.CustomerName), ct);
        return result.Match<IActionResult>(
            _ => NoContent(),
            error => error.Type == ErrorType.NotFound
                ? NotFound() : BadRequest(error.ToProblemDetails()));
    }

    [HttpDelete("{id:guid}")]
    [ProducesResponseType(StatusCodes.Status204NoContent)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public async Task<IActionResult> DeleteOrder(
        Guid id, CancellationToken ct)
    {
        var result = await sender.Send(new DeleteOrderCommand(id), ct);
        return result.IsSuccess ? NoContent() : NotFound();
    }
}

Nested Resource Controller

[ApiController]
[Route("api/orders/{orderId:guid}/items")]
[Produces("application/json")]
public sealed class OrderItemsController(ISender sender) : ControllerBase
{
    [HttpGet]
    public async Task<ActionResult<List<OrderItemResponse>>> GetItems(
        Guid orderId, CancellationToken ct)
    {
        var result = await sender.Send(
            new ListOrderItemsQuery(orderId), ct);
        return Ok(result);
    }

    [HttpPost]
    [ProducesResponseType(StatusCodes.Status201Created)]
    public async Task<IActionResult> AddItem(
        Guid orderId, AddOrderItemRequest request, CancellationToken ct)
    {
        var result = await sender.Send(
            new AddOrderItemCommand(orderId, request.ProductId,
                request.Quantity), ct);
        return CreatedAtAction(nameof(GetItems), new { orderId }, result);
    }
}

Controller Registration

// Program.cs
builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.Converters.Add(
            new JsonStringEnumConverter());
        options.JsonSerializerOptions.DefaultIgnoreCondition =
            JsonIgnoreCondition.WhenWritingNull;
    });

var app = builder.Build();

app.MapControllers();

ProblemDetails Configuration

builder.Services.AddProblemDetails(options =>
{
    options.CustomizeProblemDetails = context =>
    {
        context.ProblemDetails.Instance =
            context.HttpContext.Request.Path;
        context.ProblemDetails.Extensions["traceId"] =
            Activity.Current?.Id ??
            context.HttpContext.TraceIdentifier;
    };
});

Model Binding

// FromQuery — query string parameters
public async Task<IActionResult> Search(
    [FromQuery] string? name,
    [FromQuery] int page = 1) { }

// FromRoute — URL route parameters
[HttpGet("{id:guid}")]
public async Task<IActionResult> Get(
    [FromRoute] Guid id) { }

// FromBody — request body (default for complex types with [ApiController])
[HttpPost]
public async Task<IActionResult> Create(
    CreateOrderRequest request) { }

// FromHeader — custom headers
public async Task<IActionResult> Process(
    [FromHeader(Name = "X-Correlation-Id")] string? correlationId) { }

Anti-Patterns

• Business logic in controller actions (use handlers)
• Returning domain entities directly (use DTOs)
• Missing CancellationToken parameter
• Missing [ProducesResponseType] attributes
• Fat controllers with many responsibilities
• Constructor injection of more than ISender/IMediator

Detect Existing Patterns

1. Search for : ControllerBase or : Controller class inheritance
2. Look for [ApiController] attribute on classes
3. Check for Controllers/ folder
4. Look for services.AddControllers() in Program.cs
5. Check for [ProducesResponseType] attributes on actions

Adding to Existing Project

1. Add [ApiController] to all API controllers for automatic validation
2. Add MediatR and move logic from controller actions to handlers
3. Add [ProducesResponseType] to document response types for OpenAPI
4. Add CancellationToken parameter to all async action methods
5. Configure ProblemDetails for consistent error responses
6. Use CreatedAtAction for POST endpoints returning 201

Decision Guide

Scenario	Recommendation
Complex model binding	Controllers handle this well
Need attribute routing	Controllers with [Route]
Rapid prototyping	Minimal API may be faster
Legacy migration	Keep controllers, modernize patterns
OpenAPI generation	Both work, controllers have richer attributes

References

• https://learn.microsoft.com/en-us/aspnet/core/web-api/
• https://learn.microsoft.com/en-us/aspnet/core/mvc/controllers/actions