Question

If a requested resource is not found by the Service Layer returning null to the Web API controller; what is the best way to throw a HttpStatusCode.NotFound response back to the client without hard coding it in the controller, and by checking if it’s null?

Answer 1

Personally I would just do the checks in the controllers as per Oppositional’s comment but what you are asking for is perfectly reasonable. Again using action filters either attached per controller (or registered globally) you could do something along these lines:

Example Model:

public class Foo
{
    public string Bar { get; set; }
}

Example Controller:

public class FoosController : ApiController
{
    [NullObjectActionFilter]
    public Foo Get(string id)
    {
        // - Returns model and 200
        //return new Foo() { Bar = "TEST" };

        // - Returns 404
        //return null;
    }
}

The Filter:

public class NullObjectActionFilter : ActionFilterAttribute
{
    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        object outValue = null;
        actionExecutedContext.Response.TryGetContentValue<object>(out outValue);
        if (outValue == null)
        {
            throw new HttpResponseException(HttpStatusCode.NotFound);
        }

        base.OnActionExecuted(actionExecutedContext);

    }
}