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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. One or more Catch blocks follow it.
- 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 TryYou can list down multiple catch statements to catch a different types of exceptions in case your try block raises more than one exception in different situations.
Exception Classes in .Net Framework
Exceptions are represented as classes in the VB.Net Framework. The exception classes in the VB.Net framework are mostly derived directly or indirectly from the System.Exception class. The types System.ApplicationException and System.SystemException is some of the classes derived from System.Exception class.
The System.ApplicationException class handles errors thrown by application programs. As a result, the exceptions specified by programmers should be derived from this class.
The System.SystemException class acts as a base class for all system exceptions that are predefined.
Listed below are some predefined exception classes which are derived from the Sytem.SystemException class −
|
Exception Class |
Description |
|
System.IO.IOException |
I/O errors are handled. |
|
System.IndexOutOfRangeException |
Handles errors occur when a method refers to an array index that is out of range. |
|
System.ArrayTypeMismatchException |
Handles errors that occur when the type does not match the array type. |
|
System.NullReferenceException |
Handles problems caused by deferring to a null object. |
|
System.DivideByZeroException |
Handles problems caused by dividing a dividend by zero. |
|
System.InvalidCastException |
Handles mistakes that occur during typecasting. |
|
System.OutOfMemoryException |
Handles errors caused by a lack of free memory. |
|
System.StackOverflowException |
Handles errors caused by stack overflow. |
Handling Exceptions
In the form of try and catch blocks, VB.Net gives a structured answer to exception handling concerns. The core program statements are separated from the error-handling statements using these blocks.
Try, Catch & Finally keywords are used to implement error handling blocks.
Creating User-Defined Exceptions:
Users can also define exceptions that are derived from the ApplicationException class.
Throwing Objects
If an object is directly or indirectly derived from the 'System.Exception class,' you can throw it.
In the catch block, you can use a throw statement to throw the current object as
Throw [ expression ]
