Exception handling is an in built mechanism in .NET framework to detect and handle run time errors. The .NET framework contains lots of standard exceptions. The exceptions are anomalies that occur during the execution of a program. They can be because of user, logic or system errors. If a user (programmer) do not provide a mechanism to handle these anomalies, the .NET run time environment provide a default mechanism, which terminates the program execution.

VB.NET provides three keywords try, catch and finally to do exception handling. The try encloses the statements that might throw an exception whereas catch handles an exception if one exists. The finally can be used for doing any clean up process.

The general form try-catch-finally in VB.NET is shown below.

Try
' Statement which can cause an exception.
Catch x As Type
' Statements for handling the exception
Finally
End Try 'Any cleanup code

If any exception occurs inside the try block, the control transfers to the appropriate catch block and later to the finally block.

But in VB.NET, both catch and finally blocks are optional. The try block can exist either with one or more catch blocks or a finally block or with both catch and finally blocks. 

If there is no exception occurred inside the try block, the control directly transfers to finally block. We can say that the statements inside the finally block is executed always. Note that it is an error to transfer control out of a finally block by using break, continue, return or goto. 

In VB.NET, exceptions are nothing but objects of the type Exception. The Exception is the ultimate base class for any exceptions in VB.NET. The VB.NET itself provides couple of standard exceptions. Or even the user can create their own exception classes, provided that this should inherit from either Exception class or one of the standard derived classes of Exception class like DivideByZeroExcpetion ot ArgumentException etc. 

Uncaught Exceptions 

The following program will compile but will show an error during execution. The division by zero is a runtime anomaly and program terminates with an error message. Any uncaught exceptions in the current context propagate to a higher context and looks for an appropriate catch block to handle it. If it can't find any suitable catch blocks, the default mechanism of the .NET runtime will terminate the execution of the entire program.

//VB.NET: Exception Handling
Imports System
Class MyClient
Public Shared Sub Main()
Dim x As Integer = 0
Dim div As Integer = 100 / x
Console.WriteLine(div)
End Sub 'Main
End Class 'MyClient

The modified form of the above program with exception handling mechanism is as follows. Here we are using the object of the standard exception class DivideByZeroException to handle the exception caused by division by zero.

//VB.NET: Exception Handling
Imports System
Class MyClient
Public Shared Sub Main()
Dim x As Integer = 0
Dim div As Integer = 0
Try
div = 100 / x
Console.WriteLine("This line in not executed")
Catch de As DivideByZeroException
onsole.WriteLine("Exception occured")
End Try
Console.WriteLine("Result is {0}", div)
End Sub 'Main
End Class 'MyClient

In the above case the program do not terminate unexpectedly. Instead the program control passes from the point where exception occurred inside the try block to the catch blocks. If it finds any suitable catch block, executes the statements inside that catch and continues with the normal execution of the program statements.

If a finally block is present, the code inside the finally block will get also be executed.

//VB.NET: Exception Handling
Imports System
Class MyClient
Public Shared Sub Main()
Dim x As Integer = 0
Dim div As Integer = 0
Try
div = 100 / x
Console.WriteLine("Not executed line")
Catch de As DivideByZeroException
Console.WriteLine("Exception occured")
Finally
Console.WriteLine("Finally Block")
End Try
Console.WriteLine("Result is {0}", div)
End Sub 'Main
End Class 'MyClient

Remember that in VB.NET, the catch block is optional. The following program is perfectly legal in VB.NET.

//VB.NET: Exception Handling
Imports System
Class MyClient
Public Shared Sub Main()
Dim x As Integer = 0
Dim div As Integer = 0
Try
div = 100 / x
Console.WriteLine("Not executed line")
Finally
Console.WriteLine("Finally Block")
End Try
Console.WriteLine("Result is {0}", div)
End Sub 'Main
End Class 'MyClient

But in this case, since there is no exception handling catch block, the execution will get terminated. But before the termination of the program statements inside the finally block will get executed. In VB.NET, a try block must be followed by either a catch or finally block. 

Multiple Catch Blocks 

A try block can throw multiple exceptions, which can handle by using multiple catch blocks. Remember that more specialized catch block should come before a generalized one. Otherwise the compiler will show a compilation error.

//VB.NET: Exception Handling: Multiple catch
Imports System
Class MyClient
Public Shared Sub Main()
Dim x As Integer = 0
Dim div As Integer = 0
Try
div = 100 / x
Console.WriteLine("Not executed line")
Catch de As DivideByZeroException
Console.WriteLine("DivideByZeroException")
Catch ee As Exception
Console.WriteLine("Exception")
Finally
Console.WriteLine("Finally Block")
End Try
Console.WriteLine("Result is {0}", div)
End Sub 'Main
End Class 'MyClient

Catching all Exceptions

By providing a catch block without a brackets or arguments, we can catch all exceptions occurred inside a try block. Even we can use a catch block with an Exception type parameter to catch all exceptions happened inside the try block since in VB.NET, all exceptions are directly or indirectly inherited from the Exception class.

//VB.NET: Exception Handling: Handling all exceptions
Imports System
Class MyClient
Public Shared Sub Main()
Dim x As Integer = 0
Dim div As Integer = 0
Try
div = 100 / x
Console.WriteLine("Not executed line")
Catch
End Try
Console.WriteLine("Result is {0}", div)
End Sub 'Main
End Class 'MyClient

The following program handles all exception with Exception object.

//VB.NET: Exception Handling: Handling all exceptions
Imports System
Class MyClient
Public Shared Sub Main()
Dim x As Integer = 0
Dim div As Integer = 0
Try
div = 100 / x
Console.WriteLine("Not executed line")
Catch e As Exception
Console.WriteLine("oException")
End Try
Console.WriteLine("Result is {0}", div)
End Sub 'Main
End Class 'MyClient

Throwing an Exception

In VB.NET, it is possible to throw an exception programmatically. The 'throw' keyword is used for this purpose. The general form of throwing an exception is as follows. 

Throw exception_obj

For example the following statement throw an ArgumentException explicitly. 

Throw New ArgumentException("Exception")

//VB.NET: Exception Handling:
Imports System
Class MyClient
Public Shared Sub Main()
Try
Throw New DivideByZeroException("Invalid Division")
Catch e As DivideByZeroException
Console.WriteLine("Exception")
End Try
Console.WriteLine("LAST STATEMENT")
End Sub 'Main
End Class 'MyClient

Re-throwing an Exception

The exceptions, which we caught inside a catch block, can re-throw to a higher context by using the keyword throw inside the catch block. The following program shows how to do this.

//VB.NET: Exception Handling: Handling all exceptions
Imports System
Class [MyClass]
Public Sub Method()
Try
Dim x As Integer = 0
Dim sum As Integer = 100 / x
Catch e As DivideByZeroException
Throw
End Try
End Sub 'Method
End Class '[MyClass]
Class MyClient
Public Shared Sub Main()
Dim mc As New [MyClass]
Try
mc.Method()
Catch e As Exception
Console.WriteLine("Exception caught here")
End Try
Console.WriteLine("LAST STATEMENT")
End Sub 'Main
End Class 'MyClient

Standard Exceptions

There are two types of exceptions: exceptions generated by an executing program and exceptions generated by the common language runtime. System.Exception is the base class for all exceptions in VB.NET. Several exception classes inherit from this class including ApplicationException and SystemException. These two classes form the basis for most other runtime exceptions. Other exceptions that derive directly from System.Exception include IOException, WebException etc.

The common language runtime throws SystemException. A user program rather than the runtime throws the ApplicationException. The SystemException includes the ExecutionEngineException, StaclOverFlowException etc. It is not recommended that we catch SystemExceptions nor is it good programming practice to throw SystemExceptions in our applications.

System.OutOfMemoryException
System.NullReferenceException
Syste.InvalidCastException
Syste.ArrayTypeMismatchException
System.IndexOutOfRangeException 
System.ArithmeticException
System.DevideByZeroException
System.OverFlowException 

User-defined Exceptions

In VB.NET, it is possible to create our own exception class. But Exception must be the ultimate base class for all exceptions in VB.NET. So the user-defined exception classes must inherit from either Exception class or one of its standard derived classes.

//VB.NET: Exception Handling: User defined exceptions
Imports System
Class MyException
Inherits Exception
Public Sub New(ByVal str As String)
Console.WriteLine("User defined exception")
End Sub 'New
End Class 'MyException
Class MyClient
Public Shared Sub Main()
Try
Throw New MyException("RAJESH")
Catch e As Exception
Console.WriteLine(("Exception caught here" + e.ToString()))
End Try
Console.WriteLine("LAST STATEMENT")
End Sub 'Main
End Class 'MyClient

Design Guidelines

Exceptions should be used to communicate exceptional conditions. Don't use them to communicate events that are expected, such as reaching the end of a file. If there's a good predefined exception in the System namespace that describes the exception condition-one that will make sense to the users of the class-use that one rather than defining a new exception class, and put specific information in the message. Finally, if code catches an exception that it isn't going to handle, consider whether it should wrap that exception with additional information before re-throwing it.