Introduction of exception in vb.net

Introduction of Exception
By Lecturer Suraj Pandey CCT College

Exception are just runtime errors, the terms exception handling
and error handling have becomes interchangeable. Exceptions
occur when running a program is running. We can trap such
exception and recover from them, rather than letting them bring
your program to an inglorious end.
What is Exception

An exception is a problem that arises during the execution of
a program. An exception is a response to an exceptional
circumstance that arises while a program is running, such as
an attempt to divide by zero.
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.

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:

Unstructured Exception handling
Structured Exception handling
Types of Exception handling

Structured Exception Handling
Structured exception handling tests specific pieces of the
code and, as exceptions occur, adapts your exception-
handling code to the circumstances that caused the
exception. It is significantly faster in large applications than
unstructured exception handling and allows more flexible
response to errors as well as greater application reliability.
The Try...Catch...Finally control structure is fundamental
to structured exception handling. It tests a piece of code,
filters exceptions created by the execution of that code, and
reacts differently based on the type of thrown exception.

The Try...Catch...Finally block
Try...Catch...Finally control structures test a piece of
code and direct how the application should handle various
categories of error. Each of the structure's three constituent
parts plays a specific role in this process.

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

UNSTRUCTURED EXCEPTION
HANDLING
In unstructured exception handling, we place an On Error
GoTo statement at the beginning of a block of code, and it
handles any errors occurring within that block. When an
exception is raised in a procedure after the On Error GoTo
statement executes,the program branches to the line
argument specified in the On Error GoTo statement. The
line argument, which is a line number or line label, indicates
the exception handler location.

Unstructured error handling using On Error GoTo can
degrade application performance and result in code that is
difficult to debug and maintain. Structured error handling is
the recommended method.
Syntax:
 On Error GoTo
The parts of this statement are as follow:

a. On Error GoTo Line:
The On Error GoTo Line statement assumes that error-
handling code starts at the line specified in the required line
argument. If a run-time error occurs, control branches to the
line label or line number specified in the argument,
activating the error handler. The specified line must be in the
same procedure as the On Error GoTo Line statement;
otherwise, Visual Basic generates a compiler error.
We have to place an Exit Sub statement immediately before
the error-handling block. Otherwise, Visual Basic runs the
error-handling code when it reaches the end of the
subroutine, causing unwanted or unexpected results.By Lecturer Suraj Pandey CCT College

b. On Error Resume Next:
The On Error Resume Next statement specifies that in the
event of a run-time error control passes to the statement
immediately following the one in which the error occurred.
At that point, execution continues. On Error Resume Next
enables us to put error-handling routines where errors will
occur, rather than transferring control to another location in
the procedure.
Another variation on the Resume statement is Resume Line,
which is similar to On Error GoTo Line. Resume Line passes
control to a line we specify in the line argument. We can use
Resume Line only within an error handler

 Example:
 Module Module1
Sub Main()
Dim a, b, c As Integer
Console.WriteLine("Enter value for a:")
a = Console.ReadLine()
Console.WriteLine("Enter value for b:")
b = Console.ReadLine()
On Error GoTo Errorhandler
c = a / b
Console.WriteLine("Result: {0}", c)
Console.ReadKey()
Exit Sub
Errorhandler:
Console.WriteLine("You got an error")
Resume Next
 End Sub
End Module

c. On Error GoTo 0:
The On Error GoTo 0 statement disables any error handler
in the current procedure. If you do not include an On Error
GoTo 0 statement, the error handler is still disabled when
the procedure containing the exception handler ends.
d. On Error GoTo -1:
The On Error GoTo -1 statement disables any exception
handlers in the current procedure. If you do not include an
On Error GoTo -1 statement, the exception is automatically
disabled when its procedure ends.

Raising an Exception Intentionally
There are cases in programs where you might want to create
an exception because, although no visual basic trappable
exception has occurred, some situation may have occurred
that’s incompatible with your program’s logic. You can
create an exception intentionally raising an exception, with
the visual basic Err object’s Raise method, which is declared
this way internally in VB.NET:
Raise(ByVal Number As Integer, Optional ByVal Source As
Object= Nothing, Optional ByVal Description As object=
Nothing, optional ByVal HelpFile As object= Nothing
Optional ByVal HelpContext As Object=Nothing)

Here are the arguments for the Raise method:
Number --- Long integer that identifies the nature of the
exception
Source --- String expression naming the object or application
that generated the exception, use the form project.class
Description --- String expression describing the exception
Helpfile --- The path to the help file in which help on this
exception can be found
Helpcontext --- A context ID identifying a topic within
helpfile that provides help for the exception.

Example, here lets generate our own
exception, exception number 51:

Exception filtering in the catch
block
When you’re handling exceptions you usually want to handle
different types of exceptions differently, according to the
nature of the exception that occurred. This process is called
filtering.
 We can filter on specific classes of exceptions, which means
you have to prepare for the various exceptions you want to
handle.
Exceptions are based on the visual basic exception class
(which, like all other objects in visual basic, is based on the
object class)

The derived class itself has many derived classes, the
overflowException class, which is based on the
ArithmeticException class, which is based on the
SystemException class, which is based on the Exception
class:
System.Object
System.Exception
System.SystemException
System.ArithmeticException
System.OverflowException

USING MULTIPLE CATCH
STATEMENTS
You also can use multiple catch statements when you filter
exceptions. Here’s an example that specifically handles
overflow, invalid argument, and argument out of range
exceptions:

If you want to add a general exception handler to catch any
exceptions not filtered, you can add a catch block for the
Exception class at the end of the other Catch blocks:

The code in the Finally block, if there is one, is always executed
in a Try…. Catch…..Finally statement, even if there was no
exception, and even if you execute an Exit Try statement. This
allows you to deallocate resources and so on. Here’s an example
with a Finally block.
Use Finally

 Module Module1
Sub main()
Dim int1=0, int2=1, int3 As Integer
Try
int3=int2/int1
System.console.writeline(“The answer is {0}”, int3)
Catch e As System.OverflowException
System.console.WriteLine(“Exception: Arithmetic Overflow!”)
Catch e As System.ArgumentException
System.console.WriteLine(“Exception: Invalid argument value!”)
Catch e As System.ArgumentOutofRangeException
System.console.WriteLine(“Exception: Argument out of range!”)
Finally
System.console.WriteLine(“Execution of sensitive code” & “is complete” )
End Try
End Sub
End Module

Exception: Arithmetic overflow!
Execution of sensitive code is complete

 We can throw an exception using the Throw statement, and we can
also rethrow a caught exception using the Throw statement. Here’s an
example where we can explicitly throw an overflow exception:
 Module Modul1
Sub Main()
Try
Throw New OverflowException()
Catch e As Exception
System.console.writeline(e.message)
End Try
End Sub
Throwing an Exception

The .NET Framework provides a hierarchy of exception classes
ultimately derived from the base class Exception. Each of these
classes defines a specific exception, so in many cases we only
have to catch the exception. We can also create our own
exception classes by deriving from the Exception class.
When creating our own exceptions, it is good coding practice to
end the class name of the user-defined exception with the word
"Exception."
Throwing a Custom Exception

Module Module1
Sub Main()
Try
Throw New ApplicationException(“This is a new
exception”)
Catch e As Exception
System.console.WriteLine(e.Message)
End Try
End Sub
End Module

Introduction of exception in vb.net

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