How to return correct response on error in ASP.NET MVC 3.0 via JSON?

Question

I'm struggling to resolve this problem. On my local machine (Win7/VS2010/IIS 7.5) and another identical developer's machine, the following code returns the 500 status code, and the response text says "Could not locate user with specified email address".

When I deploy the site to my test server (Win2008 R2/IIS7.5) it returns the correct status code, but the content type is set to "text/html" and the responseText doesn't contain the message.

I've tried turning off custom errors on the server, which made no difference. Can anyone spot what the problem could be?

I have a form that is configured using the AjaxHelper.BeginForm method:

@using (Ajax.BeginForm("FindUser", new AjaxOptions {OnSuccess="findComplete", OnFailure="findFailed"}) 
{
    <fieldset>
        @Html.EditorFor(m => m.UserEmail)
        <div class="form-item button search">
            <input type="submit" value="Find"/>
        </div>
        <div id="find-err" class="message error hidden">
            <div class="contents"></div>
        </div>
    </fieldset>
}

With an error handling javascript function:

function findFailed(result) {
    var error = result.responseText;
    if (error) {
        $('#find-err .contents').text(error).slideDown();
    }
}

The controller action catches any errors and returns a message:

[HttpPost]
public ActionResult FindUser(FindUserModel model) 
{
    try
    {
        // code to find user

        if (user == null) 
        {
            throw new Exception("Could not locate user with specified email address.");
        }

        if (Request.IsAjaxRequest())
        {
            return Json(new { id = user.Id, name = user.Name }, JsonRequestBehavior.AllowGet);
        }

        model.FoundUser = user;
        return View("Details", model);
    }
    catch (Exception ex)
    {
        if (Request.IsAjaxRequest())
        {
            Response.StatusCode = 500;
            return Json(ex.Message, JsonRequestBehavior.AllowGet);
        }

        ModelState.AddModelError("UserEmail", ex.Message);
        return View(model);
    }
}

Any help would be much appreciated :)

Answer 1

Looks like IIS is eating the response and trying to do it's custom error stuff with it.

Try setting

Response.TrySkipIisCustomErrors = true

in your catch.

Alternatively set the following config value:

<httpErrors errorMode="Custom" existingResponse="PassThrough"/>