VB Exceptions

1
VB Exceptions

• The VB Exception mechanism handles errors and
  exceptional circumstances with minimal logic in
  your code
• Exceptions allow you to write code as though it
  executes without problems and uses separate
  exception handlers to deal with errors and
  exceptions that may arise during execution

2
Exceptions

• Exceptions are thrown by attempts to perform
  illegal operations, such as
• division by 0
• conversion errors
• attempts to use null object
• If not caught, these exceptions stop program
  execution

3
Exception Handling

• Exceptions can be caught and processed without
  ending the program
• Multiple exceptions can be thrown and multiple
  exceptions can be caught
• All exceptions can be caught at the same place in
  a procedure, making the rest of the code simpler

4
Exception Handling (Cont'd)

• Exceptions are passed up the execution chain
  until they are either caught by your code or by
  VB
• You may catch an exception at any level before VB
  does

5
Exception Handling (Cont'd)

• Say subroutine A calls subroutine B which calls
  subroutine C. If subroutine C throws an
  exception
• you may catch the exception in C
• you may catch the exception in B
• you may catch the exception in A
• you may leave it to VB to catch the exception,
  which ends the program

6
Exception Example

• This form asks the user to enter a number and
  dies if the user enters a non-number

7
Exception Example (Cont'd)

• If the exception is caught instead, it can be
  handled by the program and the user may try again

8
Exception Handling

• Exceptions can be caught and handled using the
  constructTry statementsCatch exception As
  exType statementsFinally statementsEnd
  Try

9
Exception Handling Construct

• Try clause contains code to be executed
• Catch clause contains the name of the exception
  to be caught and the code to deal with it
• Finally clause contains code to be executed
  whether an exception was thrown or not

10
Exception Handling (Cont'd)

• When an exception is thrown execution of the
  program is immediately transferred to the first
  catch block that can handle that exception

11
Exception Handling (Cont'd)

• Public Sub A() Try 1st Line 2nd Line
  3rd Line Catch e As Exception
• If 1st Line throws an exception then 2nd and 3rd
  Line are not executed execution moves to the
  Catch block

12
Exception Properties

• Message a user-friendly message about the
  exception that was thrown
• ToString a more detailed description of the
  exception that was thrown including its type and
  the Message
• Not very user friendly but useful to the
  programmer to see exactly what exception was
  thrown

13
Exception Example

• The number form tries to convert the contents of
  the textbox into a Decimal value
• Non numbers cause the conversion to throw an
  InvalidCastException
• This exception can be caught and an error message
  posted

14
Exception Example

• Dim n As Decimal, flag As Boolean TrueTry
  n CDec(tbMyNumber.Text)Catch ex As
  InvalidCastException flag False
  MsgBox(ex.Message)Finally If flag Then
  lblResult.Text "Valid Number" Else
  lblResult.Text "Invalid Number" End IfEnd
  Try