.NET Exceptions

In this tutorial you will learn about Exceptions, Common Exceptions, Handling Exceptions - Try Block, Catch Block, Throw Statement, Finally Block, Salient points about error handling Custom Exceptions - Managing Unhandled Exception

Exceptions

Abnormal conditions can become obstacles in the execution of very good programs. These conditions may force the program to breakdown, or halt or just go into a limbo. The Network connection may snap or a printer may run out of paper. No programmer can foresee these problems, yet he must give the user an option to gracefully save the work done up to the point and exit the program. The Exception Management Application Block provides a simple yet extensible framework for handling exceptions in VB.NET.

What is an Exception?

Programs are designed to execute normally in predefined conditions. However, the program may occasionally be confronted with an abnormal situation which it is not built to handle. Such a situation will result in an “exception” to the norm. Programs will have to be told what it has to do when it encounters such abnormal conditions. The process of anticipating and providing for handling such extraordinary situations is known as exception handling. In .Net Framework 2005 we have a facility to break, correct the code and continue in case of exception. This facility was withdrawn in the earlier releases of .Net Framework.

An exception may occur due to a logical or a syntax error. Where syntax errors may be caught by the complier, logical errors are likely to be missed Therefore, you may have a program that may be trying to delete a file that does not exist or it may be trying to divide by zero or it may just be a null pointer error or a System IO exception. The program must be told what to do and how to do it. Some of these code level bugs come to light only at the time of execution or under extreme conditions. Most programmers as a matter of caution create exceptions and raise them on some predefined conditions to monitor the working of the program. They ensure that they provide for exception handling for the most commonly encountered problems and situations. Microsoft .NET framework 2005 has made this task of the programmer easier by building into the application the Exception Assistant.

An example of an exception handler code is illustrated below for your understanding:

Click here for Sample Code

In the above example a class has been instantiated and the SecondMethod is called. In the above code two variables a and b are passed on to the SecondMethod and a process is initiated in the SUB. Once this process is complete the SUB FirstMethod is called. The expected condition here is that the value of ‘n’ should not be zero. If at this point the value of “n” is set to zero the program will breakdown or terminate.

The Exception Assistant helps the programmer troubleshoot exceptions and also helps him understand more about the exceptions.

Some of the most common exceptions are listed below:

• Code Access Security Exceptions
  
• MicroSoft.Tools.CannotRemoveControlException
  
• System.AccessViolationException
  
• System.ArrayTypeMismatchExceptionSystem.Data.OleDb.OleDbException
  
• System.Data.ConstraintException
  
• System.Data.InvalidExpressionExcepton
  
• System.Data.NoNull.AllowedException
  
• System.Data.OdbcExcepton
  
• System.Data.OracleClient.OracleException
  
• System.DivideByZeroException
  
• System.DuplicateWaitObjectException
  
• System.IO.DirectroyNotFoundException
  
• System.IO.EndOfStreamException
  
• System.IO.FileNotFoundException
  
• System.IO.IOException
  
• System.IO.PathTooLongException
  
• System.IO.InternalBufferOverflowException
  
• System.NotSupportedException
  
• NullReferenceException
  
• System.ObjectDisposedException
  
• System.OperationCalnceledException
  
• System.OutOfMemoryException
  
• System.OverflowException
  
• System.Security.SecurityException
  
• System.StackOverFlowException
  
• System.UnauthorizedException
  

Handling Exceptions

Visual Basic supports both Structured and unstructured exception handling. If an exception occurs in a method that is not equipped to handle it, the exception is propagated back to the calling method, or the previous method. If the previous method also has no exception handler, the exception is propagated back to that method's caller, and so on. The search for a handler continues up the call stack, which is the series of procedures called within the application. If it fails to find a handler for the exception, an error message is displayed and the application is terminated.

The On Error statement is used specifically for unstructured exception handling. In unstructured exception handling, On Error is placed at the beginning of a block of code. It then has "scope" over that block; it handles any errors occurring within the block. If the program encounters another On Error statement, that statement becomes valid and the first statement becomes invalid.

In the Structured Exception Handling, the blocks of code are encapsulated with each block having one or more associated handlers. Each handler specifies some form of filter condition on the type of exception it handles. When an exception is raised by code in a protected block, the set of corresponding handlers is searched in order, and the first one with a matching filter condition is executed. A single method can have multiple structured exception handling blocks, and the blocks can also be nested within each other.

Using the Try...Catch...Finally statement, you can protect blocks of code that have the potential to raise errors. You can nest exception handlers, and the variables declared in each block will have local scope.

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