Can you elaborate a bit more about what you want to accomplish. Normally I would suggest using an API controller if you want json data, which can format it automatically if you set the default formatter to a json formatter:

 public class ApiConfigurationComponent : IComponent
    {
        public void Initialize()
        {
            GlobalConfiguration.Configuration.Formatters.Clear();
            GlobalConfiguration.Configuration.Formatters.Add(new JsonMediaTypeFormatter
            {
                SerializerSettings = new JsonSerializerSettings
                {
                    ContractResolver = new CamelCasePropertyNamesContractResolver()
                }
            }); 
        }

Perhaps a surface controller(https://our.umbraco.com/documentation/reference/routing/surface-controllers)? Instead of returning an ActionResult, you could return a JSONResult. Also assuming that the data is for public consumption.

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