What we saw in the Part I
We defined the exception, then we discussed in detail about the Unstructured Exception handling.
Introduction
The application is divided into blocks of code, those blocks that have the probability of raising an error contain one or more associated exception handlers. Structured Exception handler is very simple, it is very similar to the if .... else if .... End If or
Select Case statements.
Structured Exception handling is done using Try....Catch....Finally statement.
Try....Catch....Finally Statement
When an application runs, the code in the Try block is executed first, if the code gives any error, the error is handled by the catch block. We can have multiple catch block in the Try statement. (Because, our code may raise different exception on different situation.)
For example, Let us assume that we are doing both file operation and database operation, there are changes for getting the IOExceptions, File Exceptions and Database exceptions. We need to handle all the exceptions, ie., we need to be prepared for everything (Whatever be the exception) our code should be capable of handling them.
Syntax for Try....catch statement
Try ' Block of code, where the chances of getting exception is there
catch <Exception object< 'Block of code that nneds to be executed when the exception of that type is caught
Finally 'Block of code that needs to be executed, even when there is no exceptions. End Try
Finally block is executed just before exists the Try...Catch...Finally back in which the error occured. Finaly block is executed even when the exception is not raised.
Finally block is very useful. For example, we are doing some file operations, where i have opened a file, if the file is opened, it will locked for editing the file, lock is removed only when we close the file. So we need to close the file even if the exception is raised or not.
Example for Structured Exception Handling
Public Function Structeddivide(ByVal a As Integer, ByVal b As Integer) As Integer
Try
Return a / b
Catch ex As OverflowException
MsgBox("Cannot divide by zero")
Return -1
Finally
MsgBox("Calculation over")
End Try
End Function
The above code when 0 is passed as the second parameter and when we try to execute this method it raises an OverFlowException, since any number divided by Zero will result in NaN.
Example for Catching Multiple Exceptions
Public Function Structeddivide(ByVal a As Integer, ByVal b As Integer) As Integer
Dim fis1 As System.IO.FileStream
Dim fw As System.IO.StreamWriter
Dim fis2 As System.IO.FileStream
Dim fr As System.IO.StreamReader
Try
fis1 = New System.IO.FileStream("c:\WriteResult.txt", IO.FileMode.Append)
fw = New System.IO.StreamWriter(fis1)
fw.WriteLine(a / b)
fis1 = New System.IO.FileStream("c:\WriteResult.txt", IO.FileMode.Append)
fr = New System.IO.StreamReader(fis1)
MsgBox(fr.ReadLine)
Return a / b
Catch ioexc As System.IO.IOException
MsgBox("IO Exception Occured")
Catch ex As OverflowException
MsgBox("Cannot divide by zero")
Return -1
Finally
MsgBox("Calculation over")
fw.Close()
fis1.Close()
End Try
End Function
When an error occurs in the Try block code, the control automatically transfers to the first try catch block, If the Exception matches with the exception handled in the catch block executes that block of code or else skips to the next catch block. If no matching catch block is found the control is transferred to the function or the link where this function was actually called. Regardless of whichever Catch block is executed, the code in the Finally block is executed before exiting the Try block.
Some Properties of Exception Class
HelpLink : Help File associated with the exception
Message : Read Only Property, Which gives the description of the exception raised
Source : The name of the application or object which caused the exception
StackTrace : Gives the full history of the exception, like where the exception is thrown, why it is thrown etc.,
We will see more about Nested Exceptions, creating user defined exceptions in the next section of this article
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