WebAPI controller not working while another one does - asp.net-web-api

Question

I have an API that works fine locally and when I move it to the live environment it doesn't.
The main POST action on the affected controller returns:
NotFound
With a test GET action I get back:
"Message": "No HTTP resource was found that matches the request URI
Strangely, when I uploaded a testController with the same test action as used in the main controller I get a proper response from the API.
This is the test that works fine:
public class TestController : ApiController
{
[AllowAnonymous]
[HttpGet]
public HttpResponseMessage helloWorld()
{
return Request.CreateResponse(HttpStatusCode.OK, "HelloWorld!");
}
}
The controller which does not work:
public class DeviceController : ApiController
{
[AllowAnonymous]
[HttpGet]
public HttpResponseMessage helloWorld() // This returns: "No HTTP resource was found that matches the request URI 'http://api.mySite.com/api/Device/helloWorld'."
{
return Request.CreateResponse(HttpStatusCode.OK, "HelloWorld!");
}
[AllowAnonymous]
[HttpPost]
public HttpResponseMessage Login([FromBody] LoginObject loginObject) // This returns: "NotFound"
{
...
}
}
Here is the web config:
public static class WebApiConfig
{
public static void Register(HttpConfiguration config)
{
config.MapHttpAttributeRoutes();
config.Routes.MapHttpRoute(
name: "API Default",
routeTemplate: "api/{controller}/{action}/{id}",
defaults: new { id = RouteParameter.Optional }
);
}
}

Answer 1

Try to add explicitly declare of route like by acrion
[Route("api/Device/helloWorld")]
[AllowAnonymous]
[HttpGet]
public HttpResponseMessage helloWorld()
or
[RoutePrefix("api/Device")]
public class DeviceController : ApiController
and then
[Route("helloWorld")]
[AllowAnonymous]
[HttpGet]
public HttpResponseMessage helloWorld()

Answer 2

For poor sap's like myself in the future: Ensure the methods on your controller are public.

Answer 3

I spent some time looking for the answer to this problem in .NET 7.0 after I had made a new project (which automatically created a WeatherForecastController).
It turns out that the project had also automatically created a file named proxy.conf.js. In the file, the context: setting was set to "/weatherforecast". I changed it to "/api" instead and then changed [Route("[controller]")] to [Route("api/[controller]")] in both controller files. The controllers worked fine after that.