Centralize Exception Handling in Web API

Implementation is simple but there are two problems in this implementation, the first one is :

1. We are treating all type of Exception as Internal Exception which is not true in reality.
2. We are populating whole HttpResponseException object within catch block and it will execute when only “DevideByZeroException” will occur. So, if we suspect that one function might throw 5 types of Exception we have to populate “HttpResponseException” object five times within single function which will lead towards bad solution.

Here is output of this example.



Any way this implementation is bad but not wrong, because we are getting our customized Exception message. That’s fine and cool at least we have solved one problem. Now we will solve the second problem which we have mentioned above. We will not implement try-catch in each and every controller; from any central location we will try to handle all Exceptions.

Override OnException() function from ExceptionFilterAttribute class

We can make centralize Exception handling mechanism by overriding onException() function of “ExceptionFilterAttribute” class. In this example we have override onException exception and populating object of “HttpRescponseException” object by setting exception message and HttpStatuscode property.

Just keep this class in Global.asax page of your application.

public class ExceptionAttribute : ExceptionFilterAttribute

Now, only one task is there, we have to set the class name as attribute over controller and it will release us to implement try-catch in each and every action. When any type of exception will occur, the “onException” function will execute in background. Here is implementation of controller.

using System;

namespace MVC4.Controllers

}

We did not given client code to reduce redundancy; the client code is same as above. Now, we are seeing that we are getting message relevant to exception type without implementing try-cat within each controller. This is not true only for “Values” controller, we can implement ‘n’ number of controller without try-cat block and “onException” function will take care about all exception. We somehow we have centralize exception handling system.

Now the is one problem in this implementation. We are treating all Exception as InternalException again. There is lack of proper categorization of Exceptions. We will solve this limitation shortly. But before that we will look “onException” function bit closely. How the magic is happening?

When we are setting class name (in this example [ExceptionAttribute]) as attribute of any action MVC framework internally wrap the whole action within try block and put “HttpResponseException” object generation mechanism within catch block. The pseudo code representation is like below.

Try
{
//content of particular controller
}
Catch
{
//HttpResponseException message generation mechanism.
}

Ok, this is the internal operation behind this magic. Now we will solve the issue with exception categorization and for that we will maintain one dictionary to maintain all Exception type and it’s “HttpResponsecode” . Now when exception will come we will check with dictionary entry if found we will populate Exception message with relevant information if not found then we will treat the Exception as anonymous exception. Here is sample implementation.

public class ExceptionAttribute : ExceptionFilterAttribute