Throw Statement

Definition:
Specifies that a named exception should be generated and thrown from the current execution point.

Syntax:
Throw [<exceptionname>[(args)]]

Syntax Description

Throw Keyword used to generate exceptions.

exceptionname Name of exception to throw.

args Optional arguments that the named exception includes.

Details:
The Throw statement is used by program code to signal errors when the program has determined that something has gone wrong and execution cannot continue from that particular point. For example, assume that a file read operation must first open a file before reading. If a file cannot be opened, the procedure would "throw" an exception. Simply continuing at this point, would cause problems. Similarly, a return value cannot always be used to indicate a failure. For example, using the file read example but requiring the operation to return any valid integer instead, would create a situation where there is no value that could be used to signal an error.

The other standard method used to signal errors is to set an object property that can be checked after calling a function. This method, however, burdens the caller with a lot of error checking responsibilities. The recommended method for dealing with unexpected errors is to throw exceptions, which forces the exceptional condition to be dealt with by qualified error handlers.

There is a difference between exceptions that the system generates and ones that interpretive code generates with the Throw statement. This distinction allows the integrated debugger to trap the exceptions independently and suspend on throws of system exceptions, but not on throws of interpretive exceptions.

To "rethrow" an exception that has been caught in a text block, use the Throw statement without arguments. Doing this causes the same exception to roll back through the code (through each stack frame) until it finds the next matching handler for the exception. If no matching handler is found, the default handler is triggered.

See Also:
Try...Catch Statements

Example:
The Sub "Test" illustrates using an Interpretive Try..Catch block to guard against certain interpretive exceptions, namely the FileNotFound exception. When this exception occurs as a result of executing the "GetInteger" function the program will respond by posting an InfoBox.

The Function "GetInteger" shows how one exception (e.g. a system exception) can be turned into an interpretive exception. This is done with a Try..Catch block that catches the "FileError" exception and responds by throwing the "FileNotFound" exception. This is done to generate a custom exception that has more information, like the filename in this example.

Sub Test()
Try
Dim result as Integer
result = GetInteger()
Catch FileNotFound(fn As String)
InfoBox "File could not be opened: " & fn
End Try
End Sub

Function GetInteger() As Integer
Try
TextFile.FileName = "demo.txt"
TextFile.Open(True)
GetInteger = 99
Catch FileError
Throw FileNotFound("demo.txt")
End Function