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Working with Exceptions - Part I (Unstructed Exception Handling).
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In this section of article I will be telling about Exceptions, types of exceptions and I will tell in details about the Unstructured exception handling.
Introduction
An exception is termed as an abnormal condition encountered by an application during execution.
Exception Handling is the process of providing an alternate path to be executed when the application is unable to execute in the desired way.
Is this Exception Handling Necessary ?
- By Handling exceptions, we can the prevent the termination of our application.
For example, Most of the project involves working with databases. Suppose, let us assume that we are working in a web application, where we have one login page, which takes the user name and password, checks with the database for authorizing the user.
Now if the Database Server is down, we will be getting an error (Exception) stating that the db server is down, that will be displayed to the user if not handled.
Using Exception Handling we can capture the Exception or error when it is occurred and act accordingly. ie., displaying a Error page, saying that
"Sorry! Your request cannot be processed at this moment, Please try again after some time"
There are two ways of handling the exceptions:
- Structured Exception Handling
- UnStructured Exception Handling
First, lets have a quick look at the Unstructured Exception Handling.
Unstructed Exception Handling
Unstructed exception handling is carried out using the On Error Statement. The On Error statement handles any error that occurs in that block of code and transfer the control to the line label.
Syntax
On Error GoTo <line Label>
or
On error Resume Next (This statement is used to specify that when an error occurs, the control should pass to the next line of code followin the line in which the error occured.)
Note: When an error occurs in a procedure which doesn't have any On Error or Try ..... Catch ... Finally block, the exception or error is passed back to the calling procedure and the error is handled in the calling procedure.
Example of Unstructed exception handling
Public Function unstructeddivide( ByVal a As Integer, ByVal b As Integer) As Integer
On Error Resume Next
Return a / b
End Function
The above code snippet has a chance of raising an error if we pass 0 for the second value, so i have used to on error resume next statement, which ignores the error and proceeds with the next statement.
Syntax/Format of Unstructured exception handling
Public sub FunctionName()
On Error GoTo LineLabel
'the block of code which may raise an error
Exit Sub
LineLabel:
'The block of code which is executed when error occurs.
Resume
End Sub
Explanation
When an error occurs in the code wrriten in the On Error GoTo block, the control passes to the error handler LineLabel. Then the code in the error handling block is executed, When the control is transferred to the Resume statement, control return back to the line of code in which the error
occured.
The Exit Sub statement should be placed immediately before the error handler block. If the Exit Sub statement is not written, the error handler code is executed even if there is no error/exception condition giving unexpected results.
Public Function unstructeddivide(ByVal a As Integer, ByVal b As Integer) As Integer
On Error GoTo linelab1
unstructeddivide = (a / b)
Exit Function
linelab1:
MsgBox("Divide by zero not allowed")
Resume Next
End Function
Note:
- If there is any probability for an error to raise in a function, it is
better to call On Error Resume Next statement.
- On Error GoTo 0 statement turns
off the process of error trapping and disables the error handler.
Conclusion
Unstructured exception handling is not recommended, since the code written
using On Error Statement is very difficult to maintain. Better
stick with Structed Exception Handling.
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