Returning a 404 from an explicitly typed ASP.NET Core API controller (not IActionResult)

This is addressed in ASP.NET Core 2.1 with ActionResult<T>:

public ActionResult<Thing> Get(int id) {
    Thing thing = GetThingFromDB();

    if (thing == null)
        return NotFound();

    return thing;
}

Or even:

public ActionResult<Thing> Get(int id) =>
    GetThingFromDB() ?? NotFound();

I'll update this answer with more detail once I've implemented it.

Original Answer

In ASP.NET Web API 5 there was an HttpResponseException (as pointed out by Hackerman) but it's been removed from Core and there's no middleware to handle it.

I think this change is due to .NET Core - where ASP.NET tries to do everything out of the box, ASP.NET Core only does what you specifically tell it to (which is a big part of why it's so much quicker and portable).

I can't find a an existing library that does this, so I've written it myself. First we need a custom exception to check for:

public class StatusCodeException : Exception
{
    public StatusCodeException(HttpStatusCode statusCode)
    {
        StatusCode = statusCode;
    }

    public HttpStatusCode StatusCode { get; set; }
}

Then we need a RequestDelegate handler that checks for the new exception and converts it to the HTTP response status code:

public class StatusCodeExceptionHandler
{
    private readonly RequestDelegate request;

    public StatusCodeExceptionHandler(RequestDelegate pipeline)
    {
        this.request = pipeline;
    }

    public Task Invoke(HttpContext context) => this.InvokeAsync(context); // Stops VS from nagging about async method without ...Async suffix.

    async Task InvokeAsync(HttpContext context)
    {
        try
        {
            await this.request(context);
        }
        catch (StatusCodeException exception)
        {
            context.Response.StatusCode = (int)exception.StatusCode;
            context.Response.Headers.Clear();
        }
    }
}

Then we register this middleware in our Startup.Configure:

public class Startup
{
    ...

    public void Configure(IApplicationBuilder app)
    {
        ...
        app.UseMiddleware<StatusCodeExceptionHandler>();

Finally actions can throw the HTTP status code exception, while still returning an explicit type that can easily be unit tested without conversion from IActionResult:

public Thing Get(int id) {
    Thing thing = GetThingFromDB();

    if (thing == null)
        throw new StatusCodeException(HttpStatusCode.NotFound);

    return thing;
}

This keeps the explicit types for the return values and allows easy distinction between successful empty results (return null;) and an error because something can't be found (I think of it like throwing an ArgumentOutOfRangeException).

While this is a solution to the problem it still doesn't really answer my question - the designers of the Web API build support for explicit types with the expectation that they would be used, added specific handling for return null; so that it would produce a 204 rather than a 200, and then didn't add any way to deal with 404? It seems like a lot of work to add something so basic.

You can actually use IActionResult or Task<IActionResult> instead of Thing or Task<Thing> or even Task<IEnumerable<Thing>>. If you have an API that returns JSON then you can simply do the following:

[Route("api/[controller]")]
public class ThingsController : Controller
{
    // GET api/things
    [HttpGet]
    public async Task<IActionResult> GetAsync()
    {
    }

    // GET api/things/5
    [HttpGet("{id}")]
    public async Task<IActionResult> GetAsync(int id)
    {
        var thingFromDB = await GetThingFromDBAsync();
        if (thingFromDB == null)
            return NotFound();

        // Process thingFromDB, blah blah blah
        return Ok(thing); // This will be JSON by default
    }

    // POST api/things
    [HttpPost]
    public void Post([FromBody] Thing thing)
    {
    }
}

Update

It seems as though the concern is that being explicit in the return of an API is somehow helpful, while it is possible to be explicit it is in fact not very useful. If you're writing unit tests that exercise the request / response pipeline you are typically going to verify the raw return (which would most likely be JSON, i.e.; a string in C#). You could simply take the returned string and convert it back to the strongly typed equivalent for comparisons using Assert.

This seems to be the only shortcoming with using IActionResult or Task<IActionResult>. If you really, really want to be explicit and still want to set the status code there are several ways to do this - but it is frowned upon as the framework already has a built-in mechanism for this, i.e.; using the IActionResult returning method wrappers in the Controller class. You could write some custom middleware to handle this however you'd like, however.

Finally, I would like to point out that if an API call returns null according to W3 a status code of 204 is actually accurate. Why on earth would you want a 404?

204

The server has fulfilled the request but does not need to return an entity-body, and might want to return updated metainformation. The response MAY include new or updated metainformation in the form of entity-headers, which if present SHOULD be associated with the requested variant.

If the client is a user agent, it SHOULD NOT change its document view from that which caused the request to be sent. This response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view, although any new or updated metainformation SHOULD be applied to the document currently in the user agent's active view.

The 204 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.

I think the first sentence of the second paragraph says it best, "If the client is a user agent, it SHOULD NOT change its document view from that which caused the request to be sent". This is the case with an API. As compared to a 404:

The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.

The primary difference being one is more applicable for an API and the other for the document view, i.e.; the page displayed.

In order to accomplish something like that(still, I think that the best approach should be using IActionResult), you can follow, where you can throw an HttpResponseException if your Thing is null:

// GET api/things/5
[HttpGet("{id}")]
public async Task<Thing> GetAsync(int id)
{
    Thing thingFromDB = await GetThingFromDBAsync();
    if(thingFromDB == null){
        throw new HttpResponseException(HttpStatusCode.NotFound); // This returns HTTP 404
    }
    // Process thingFromDB, blah blah blah
    return thing;
}