I had the same issue. I'm running a java rest app on a jboss server. But I think the solution is similar on an ASP .NET webapp.
Firefox makes a pre call to your server / rest url to check which options are allowed. That is the "OPTIONS" request which your server doesn't reply to accordingly. If this OPTIONS call is replied correct a second call is performed which is the actual "POST" request with json content.
This only happens when performing a cross-domain call. In your case calling 'http://localhost:16329/Hello' instead of calling a url path under the same domain '/Hello'
If you intend to make a cross domain call you have to enhance your rest service class with an annotated method the supports a "OPTIONS" http request. This is the according java implementation:
@Path("/rest")
public class RestfulService {
@POST
@Path("/Hello")
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.TEXT_PLAIN)
public string HelloWorld(string name)
{
return "hello, " + name;
}
//THIS NEEDS TO BE ADDED ADDITIONALLY IF MAKING CROSS-DOMAIN CALLS
@OPTIONS
@Path("/Hello")
@Produces(MediaType.TEXT_PLAIN+ ";charset=utf-8")
public Response checkOptions(){
return Response.status(200)
.header("Access-Control-Allow-Origin", "*")
.header("Access-Control-Allow-Headers", "Content-Type")
.header("Access-Control-Allow-Methods", "POST, OPTIONS") //CAN BE ENHANCED WITH OTHER HTTP CALL METHODS
.build();
}
}
So I guess in .NET you have to add an additional method annotated with
[WebInvoke(
Method = "OPTIONS",
UriTemplate = "Hello",
ResponseFormat = WebMessageFormat.)]
where the following headers are set
.header("Access-Control-Allow-Origin", "*")
.header("Access-Control-Allow-Headers", "Content-Type")
.header("Access-Control-Allow-Methods", "POST, OPTIONS")