Cant get ASP.NET MVC 6 Controller to return JSON

First of all you can use IEnumerable<Order> or IEnumerable<object> as return type instead of JsonResult and return just orderRepository.GetAll(). I recommend you to read the article fr additional information.

About another error with Bad Gateway. Try to add Newtonsoft.Json in the latest version 8.0.2 to dependencies in package.json and to use use

services.AddMvc()    .AddJsonOptions(options => {        options.SerializerSettings.ReferenceLoopHandling =            Newtonsoft.Json.ReferenceLoopHandling.Ignore;    });

By the way one can reproduce the error "HTTP Error 502.3 - Bad Gateway", which you describes if I just set breakpoint on the return statement of working code and wait long enough. Thus you will see the error "HTTP Error 502.3 - Bad Gateway" very soon on many common errors.

You can consider to us more helpful serialization options. For example

services.AddMvc()    .AddJsonOptions(options => {        // handle loops correctly        options.SerializerSettings.ReferenceLoopHandling =            Newtonsoft.Json.ReferenceLoopHandling.Ignore;        // use standard name conversion of properties        options.SerializerSettings.ContractResolver =            new CamelCasePropertyNamesContractResolver();        // include $id property in the output        options.SerializerSettings.PreserveReferencesHandling =            PreserveReferencesHandling.Objects;    });