Many service responses are returned as JSON objects. For this, the application must implement some basic configuration and error handling in each method. By utilising the cross-cutting facet of attributes, this code is unified into a common location and applied wherever required.
Signalling
Firstly, the action method that returns JSON has to be marked. The JsonHandlerAttribute class inherits from ActionFilterAttribute. It is applied to the method in question. When the method is invoked, the ASP.NET pipeline executes the OnActionExecuted method on its attribute. The method converts the System.Web.Mvc.JsonResult result into a custom-written class called FitNet.Web.Infrastructure.JsonNetResult. This class inherits from JsonResult and adds some shared functionality.
Error Handling and Serialization
The conversion from JsonResult to JsonNetResult is performed inside the JsonHandlerAttribute class. The Result property of the current filter context is typed into JsonNetResult. If the conversion is successful, then a new instance of JsonNetResult is created and the copies of the current result copied over.
Finally, the ExecuteResult method is invoked on the newly created result instance. This step performs the following error checks.
- Deny GET requests by default, unless it is explicitly allowed in the method.
- Set the content type of the response to
application/json. - Configure the underlying serializer to throw an error in case of looping references.
If all the checks pass, then the response data is serialized and the output is assigned to the Output property of the current response.
This structure of the code removes the need to have all these checks in each method that has to return a JSON response.