Introduction

Enhanced Exception Handling is one of key features that is most prominently available in .Net both 2003 version and Whidbey versions of VB.Net.

This paper discusses the implementation of Custom Exception Handling using the existing features of VB.Net.

The Concept

The whole idea of having customized Exception Handling is centered around the fact that there needs to be a generic approach of catching and throwing Exceptions.

For implementing custom Exception Handling we need to derive the class CustomException from the system base class ApplicationException. In general, for customizing Exception Handling the following components are necessary:

   1. A custom exception class which would host the exception thrown and can be thrown as a new exception.
   2. A message class that would host all user - friendly Application messages that need to be displayed.

Implementation

The following are the steps that are required to implement custom exception for creation of the above mentioned components:

Step 1:

Define a project called Enumerators that would contain the definitions of all the Enumerators as the following:

Imports System
Namespace CustomException_
'/ <summary>
'/ Severity level of Exception
'/ </summary>
Public Enum SeverityLevel
Fatal
Critical
Information
End Enum 'SeverityLevel

'/ <summary>
'/ Log level of Exception
'/ </summary>
Public Enum LogLevel
Debug
[Event]
End Enum 'LogLevel
End Namespace 'CustomException

One advantage of creating a custom Exception class is that the Constructor can be enabled to writing to a Log on instantiation of the CustomException Object using TraceListeners. The entry to the log would be based on the logLevel. This would force - write an entry each time the custom Exception is thrown.

Thus we have a customException which could be thrown in the catch-handlers of system-defined exceptions.

Step 3:

For implementing the CustomMessage component , create an Exception.resx File that would host the error string and the corresponding message string as key-value pair.

Ex. "Test", "Testing Error"

Step 4:

Add to project a CustomMessage.cs class File. This File would look in the following way:

Imports System
Imports System.Globalization
Imports System.Threading
Imports System.Resources
Imports System.Reflection
Namespace CustomException
'/ <summary>
'/ Summary description for CustomMessage.
'/ </summary>
Public Class CustomMessage
Public Sub New()
End Sub 'New

Public Function GetString(ByVal key As String) As String
' Create a resource manager to retrieve resources.
Dim rm As New ResourceManager("Exceptions", [Assembly].GetExecutingAssembly())
' Get the culture of the currently executing thread.
' The value of ci will determine the culture of
' the resources that the resource manager retrieves.
Dim ci As CultureInfo = Thread.CurrentThread.CurrentCulture
' Retrieve the value of the string resource and return it
Dim str As [String] = rm.GetString(key, ci)
Return str
End Function 'GetString
End Class 'CustomMessage
End Namespace 'CustomException

The GetString() of the CustomMessage class is used to retrieve the value string corresponding to the key passed as parameter from the Exceptions resource File.

Usage

a) Add to existing Solution a Test Windows Applications Project.

b) Add the References to both the CustomException and the Enumerators projects.

c) Write a function which would throw a system exception , encapsulated in a try-catch block as follows :