Exceptions provide a way to transfer control from one part of a program to another. VB.Net exception handling is built upon four keywords: Try, Catch, Finally and Throw.

• Try: A Try block identifies a block of code for which particular exceptions will be activated. It's followed by one or more Catch blocks.




• Catch: A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The Catch keyword indicates the catching of an exception.




• Finally: The Finally block is used to execute a given set of statements, whether an exception is thrown or not thrown. For example, if you open a file, it must be closed whether an exception is raised or not.




• Throw: A program throws an exception when a problem shows up. This is done using a Throw keyword.

Syntax

Assuming a block will raise an exception, a method catches an exception using a combination of the Try and Catch keywords. A Try/Catch block is placed around the code that might generate an exception. Code within a Try/Catch block is referred to as protected code, and the syntax for using Try/Catch looks like the following:

Try

    [ tryStatements ]

    [ Exit Try ]

[ Catch [ exception [ As type ] ] [ When expression ]

    [ catchStatements ]

    [ Exit Try ] ]

[ Catch ... ]

[ Finally

    [ finallyStatements ] ]

End Try

You can list down multiple catch statements to catch different type of exceptions in case your try block raises more than one exception in different situations.

Exception Classes in .Net Framework

In the .Net Framework, exceptions are represented by classes. The exception classes in .Net Framework are mainly directly or indirectly derived from the System.Exception class. Some of the exception classes derived from the System.Exception class are the System.ApplicationException and System.SystemException classes.

The System.ApplicationException class supports exceptions generated by application programs. So the exceptions defined by the programmers should derive from this class.

The System.SystemException class is the base class for all predefined system exception.

The following table provides some of the predefined exception classes derived from the Sytem.SystemException class:

Handling Exceptions

VB.Net provides a structured solution to the exception handling problems in the form of try and catch blocks. Using these blocks the core program statements are separated from the error-handling statements.

These error handling blocks are implemented using the Try, Catch and Finally keywords. Following is an example of throwing an exception when dividing by zero condition occurs:

Module exceptionProg

   Sub division(ByVal num1 As Integer, ByVal num2 As Integer)

      Dim result As Integer

      Try

          result = num1 \ num2

      Catch e As DivideByZeroException

          Console.WriteLine("Exception caught: {0}", e)

      Finally

          Console.WriteLine("Result: {0}", result)

      End Try

   End Sub

   Sub Main()

      division(25, 0)

      Console.ReadKey()

  End Sub

End Module

When the above code is compiled and executed, it produces the following result:

Exception caught: System.DivideByZeroException: Attempted to divide by zero. 

at ...

Result: 0

Creating User-Defined Exceptions

You can also define your own exception. User-defined exception classes are derived from the ApplicationException class. The following example demonstrates this:

Module exceptionProg

   Public Class TempIsZeroException : Inherits ApplicationException

      Public Sub New(ByVal message As String)

          MyBase.New(message)

      End Sub

   End Class

   Public Class Temperature

      Dim temperature As Integer = 0

      Sub showTemp()

          If (temperature = 0) Then

              Throw (New TempIsZeroException("Zero Temperature found"))

          Else

              Console.WriteLine("Temperature: {0}", temperature)

          End If

      End Sub

   End Class

   Sub Main()

      Dim temp As Temperature = New Temperature()

      Try

          temp.showTemp()

      Catch e As TempIsZeroException

          Console.WriteLine("TempIsZeroException: {0}", e.Message)

      End Try

      Console.ReadKey()

   End Sub

End Module

When the above code is compiled and executed, it produces the following result:

TempIsZeroException: Zero Temperature found

Throwing Objects

You can throw an object if it is either directly or indirectly derived from the System.Exception class.

You can use a throw statement in the catch block to throw the present object as:

Throw [ expression ]

The following program demonstrates this:

Module exceptionProg

   Sub Main()

      Try

          Throw New ApplicationException("A custom exception _

		  is being thrown here...")

      Catch e As Exception

          Console.WriteLine(e.Message)

      Finally

          Console.WriteLine("Now inside the Finally Block")

      End Try

      Console.ReadKey()

   End Sub

End Module

When the above code is compiled and executed, it produces the following result:

A custom exception is being thrown here...

Now inside the Finally Block