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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, called 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. (If source is not specified, the name of the current Visual Basic project is used.)

  • 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.

When setting the exception number for a custom exception, note that Visual Basic reserves a certain range of exceptions for itself. To make sure your custom exception doesn't conflict with that range, add the value vbObjectError to your exception number, then subtract it when you handle the exception to recover your original exception number. Let's see an example. Here, I'll generate our own exception, exception number 51:

Module Module1
    Sub Main()
        On Error Goto Handler
        Err.Raise(vbObjectError + 51)
        System.Console.WriteLine("Program completed...")
        System.Console.WriteLine("Error number {0} occurred: {1}", _
            Err.Number - vbObjectError, Err.Description)
    End Sub
End Module

You can handle these custom exceptions in the way that makes sense for your application. Here's what you see when you run this console application:

Error number 51 occurred: Application-defined or object-defined error.

You can also use the Visual Basic Error statement to raise an error like this: Error errnumber. However, the Error function is considered obsolete now, replaced by the Raise method of the Err object.

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