In this blogpost I just want to show you quickly how to configure ASP.NET WebAPI to send data as JSON
Here in the first part you can see clearing all formatters first and then add the JsonMediaTypeFormatter as the only formatter WebApi knows for my application. This makes sure I am only using JSON-format.
If you now sending requests to the API it should return the data as json.
Complete sample Startup:
Consider that if a client is sending the Accept-Header in the request with a value your API does not support it will get no data. Although JSON is the de facto standard for REST-APIs you should always remember to have your API as easy and intuitive as possible to consume for your clients. So maybe if your API goes public think about having XML and JSON as formatters.