Return JSON with error status code MVC

There is a very elegant solution to this problem, just configure your site via web.config:

<system.webServer>
    <httpErrors errorMode="DetailedLocalOnly" existingResponse="PassThrough"/>
</system.webServer>

Source: https://serverfault.com/questions/123729/iis-is-overriding-my-response-content-if-i-manually-set-the-response-statuscode

The neatest solution I've found is to create your own JsonResult that extends the original implementation and allows you to specify a HttpStatusCode:

public class JsonHttpStatusResult : JsonResult
{
    private readonly HttpStatusCode _httpStatus;

    public JsonHttpStatusResult(object data, HttpStatusCode httpStatus)
    {
        Data = data;
        _httpStatus = httpStatus;
    }

    public override void ExecuteResult(ControllerContext context)
    {
        context.RequestContext.HttpContext.Response.StatusCode = (int)_httpStatus;
        base.ExecuteResult(context);
    }
}

You can then use this in your controller action like so:

if(thereWereErrors)
{
    var errorModel = new { error = "There was an error" };
    return new JsonHttpStatusResult(errorModel, HttpStatusCode.InternalServerError);
}

I found the solution here

I had to create a action filter to override the default behaviour of MVC

Here is my exception class

class ValidationException : ApplicationException
{
    public JsonResult exceptionDetails;
    public ValidationException(JsonResult exceptionDetails)
    {
        this.exceptionDetails = exceptionDetails;
    }
    public ValidationException(string message) : base(message) { }
    public ValidationException(string message, Exception inner) : base(message, inner) { }
    protected ValidationException(
    System.Runtime.Serialization.SerializationInfo info,
    System.Runtime.Serialization.StreamingContext context)
        : base(info, context) { }
}

Note that I have constructor which initializes my JSON. Here is the action filter

public class HandleUIExceptionAttribute : FilterAttribute, IExceptionFilter
{
    public virtual void OnException(ExceptionContext filterContext)
    {
        if (filterContext == null)
        {
            throw new ArgumentNullException("filterContext");
        }
        if (filterContext.Exception != null)
        {
            filterContext.ExceptionHandled = true;
            filterContext.HttpContext.Response.Clear();
            filterContext.HttpContext.Response.TrySkipIisCustomErrors = true;
            filterContext.HttpContext.Response.StatusCode = (int)System.Net.HttpStatusCode.InternalServerError;
            filterContext.Result = ((ValidationException)filterContext.Exception).myJsonError;
        }
    }

Now that I have the action filter, I will decorate my controller with the filter attribute

[HandleUIException]
public JsonResult UpdateName(string objectToUpdate)
{
   var response = myClient.ValidateObject(objectToUpdate);
   if (response.errors.Length > 0)
     throw new ValidationException(Json(response));
}

When the error is thrown the action filter which implements IExceptionFilter get called and I get back the Json on the client on error callback.