Appendix O

Useful Exception Classes

When your program throws an exception, it’s easy enough to use a TryCatch block to catch the exception and examine it to determine its class. When you want to throw your own exception, however, you must know what exception classes are available so that you can pick the right one.

For more information on error handling, see Chapter 18, “Error Handling,” and Appendix F.

STANDARD EXCEPTION CLASSES

The following table lists some of the most useful exception classes in Visual Basic .NET. You can use one of these when you need to throw an error.

Use the Throw statement to raise an exception. The following code throws a DivideByZeroException:

Throw New DivideByZeroException("No employees are defined.")

This code passes the exception class’s constructor a message describing the exception. In this case, the divide by zero exception occurred because the application did not have any employees defined. Notice that the message explains the reason for the exception, not the mere fact that a division by zero occurred.

CUSTOM EXCEPTION CLASSES

To define a custom exception class, make a class that inherits from an exception class. To give developers who use the class the most flexibility, provide four constructors that delegate their work to the parent class’s corresponding constructors.

The following code shows the InvalidWorkAssignmentException class. The parameterless constructor passes the Exception class’s constructor a default error message. The other constructors simply pass their arguments to the Exception class’s other constructors.

Public Class InvalidWorkAssignmentException
    Inherits ApplicationException
 
    Public Sub New()
        MyBase.New("This work assignment is invalid")
    End Sub
 
    Public Sub New(msg As String)
        MyBase.New(msg)
    End Sub
 
    Public Sub New(msg As String, inner_exception As Exception)
        MyBase.New(msg, inner_exception)
    End Sub
 
    Public Sub New(info As SerializationInfo, context As StreamingContext)
        MyBase.New(info, context)
    End Sub
End Class

For more information on custom exception classes, see Chapter 18 and the online documentation for topics such as “Designing Custom Exceptions” (http://msdn.microsoft.com/ms229064.aspx) and “Design Guidelines for Exceptions” (http://msdn.microsoft.com/ms229014.aspx), or search the web for articles such as “Custom Exceptions in VB 2005” by Josh Fitzgerald (http://www.developer.com/net/vb/article.php/3590931).