What is the proper way to send an HTTP 404 response from an ASP.NET MVC action?

Shooting from the hip (cowboy coding ;-)), I’d suggest something like this:

Controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        return new HttpNotFoundResult("This doesn't exist");
    }
}

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HttpNotFoundResult:

using System;
using System.Net;
using System.Web;
using System.Web.Mvc;

namespace YourNamespaceHere
{
    /// <summary>An implementation of <see cref="ActionResult" /> that throws an <see cref="HttpException" />.</summary>
    public class HttpNotFoundResult : ActionResult
    {
        /// <summary>Initializes a new instance of <see cref="HttpNotFoundResult" /> with the specified <paramref name="message"/>.</summary>
        /// <param name="message"></param>
        public HttpNotFoundResult(String message)
        {
            this.Message = message;
        }

        /// <summary>Initializes a new instance of <see cref="HttpNotFoundResult" /> with an empty message.</summary>
        public HttpNotFoundResult()
            : this(String.Empty) { }

        /// <summary>Gets or sets the message that will be passed to the thrown <see cref="HttpException" />.</summary>
        public String Message { get; set; }

        /// <summary>Overrides the base <see cref="ActionResult.ExecuteResult" /> functionality to throw an <see cref="HttpException" />.</summary>
        public override void ExecuteResult(ControllerContext context)
        {
            throw new HttpException((Int32)HttpStatusCode.NotFound, this.Message);
        }
    }
}
// By Erik van Brakel, with edits from Daniel Schaffer :)

Using this approach you comply to the framework standards. There already is a HttpUnauthorizedResult in there, so this would simply extend the framework in the eyes of another developer maintaining your code later on (you know, the psycho who knows where you live).

You could use reflector to take a look into the assembly to see how the HttpUnauthorizedResult is achieved, because I don’t know if this approach misses anything (it seems too simple almost).

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I did use reflector to take a look at the HttpUnauthorizedResult just now. Seems they’re setting the StatusCode on the response to 0x191 (401). Although this works for 401, using 404 as the new value I seem to be getting just a blank page in Firefox. Internet Explorer shows a default 404 though (not the ASP.NET version). Using the webdeveloper toolbar I inspected the headers in FF, which DO show a 404 Not Found response. Could be simply something I misconfigured in FF.

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This being said, I think Jeff’s approach is a fine example of KISS. If you don’t really need the verbosity in this sample, his method works fine as well.