Error handling does not involve finding errors in your scripts. Instead, use error handling techniques to allow your program to continue executing even though a potentially fatal error has occurred. Ordinarily, all runtime errors that are generated by the VBScript engine are fatal, since execution of the current script is halted when the error occurs. Error handling allows you to inform the user of the problem and either halt execution of the program or, if it is prudent, continue executing the program.
There are two main elements to error handling in VBScript. The first
which informs the VBScript engine of your intention to handle errors
yourself, rather than to allow the VBScript engine to display a
typically uninformative error message and halt the program. This is
done by inserting a statement like the following at the start of a
On Error Resume Next
This tells the VBScript engine that, should an error occur, you want it to continue executing the program starting with the line of code which directly follows the line in which the error occurred. For example, in the simple WSH script:
On Error Resume Next x = 10 y = 0 z = x / y Alert z
a “Cannot divide by Zero” error is generated on the
fourth line of code because the value of
is 0. But because you’ve placed the
statement in line 1, program execution continues with line 5. The
problem with this is that when an error is generated, the user is
unaware of it; the only indication that an error has occurred is the
blank Alert box (from line 5) that’s displayed for the user.
is valid until another
statement in the line of execution is encountered. This means that if
Function A contains an
On Error statement, and
Function A calls Function B, but Function B does not contain an
Error statement, the error
handling from Function A is still valid. Therefore, if an error
occurs in Function B, it is the
Error statement in Function A that handles the
error; in other words, when an error is encountered in Function B,
program flow will immediately jump to the line of code that followed
the call to Function B in Function A. When Function A completes
statement it contains also goes out of scope. This means that, if the
routine that called Function A did not include an
Error statement, no error
handling is in place.
This is where the second element of VBScript’s error handling
comes in. VBScript includes an error object, named Err, which, when
used in conjunction with
Next, adds much more functionality to error
handling, allowing you to build robust programs and relatively
sophisticated error handling routines.
The Err object is part of the VBScript
language and contains information about the last error to occur. By
checking the properties of the Err object after a particular piece of
code has executed, you can determine whether an error has occurred
and, if so, which one. You can then decide what to do about the error
—you can, for instance, continue execution regardless of the
error, or you can halt execution of the program. The main point here
is that error handling using
On Error and the Err
object puts you in control of errors, rather than allowing an error
to take control of the program (and bring it to a grinding halt). To
see how the Err object works and how you can use it within an error
handling regimen within your program, let’s begin by taking a
look at its properties and methods.
Like all object properties, the properties of the Err object can be accessed by using the name of the object, Err, the dot (or period) delimiter, and the property name. The Err object supports the following properties:
The Number property is an integer value that
contains an error code value between
and 65535, representing the last error. If the value of
Err.Number is 0, no error has occurred.
The line of code like the following, then, can be used to determine
if an error has occurred:
If Err.Number <> 0 Then
Although the properties of the Err object provide information on the
last error to occur in a script, they do not do so permanently. All
the Err object properties, including the Number property, are set
either to zero or to zero-length strings after an End Sub, End
Function, Exit Sub or Exit Function statement. In addition, though,
you can explicitly reset
zero after an error by calling the Err object’s
Clear method. The WSH script in Example 4.8 illustrates the importance of resetting the
object after an error occurs.
Example 4-8. Failing to Reset the Err Object
Dim x, y ,z On Error Resume Next x = 10 y = 0 z = x / y If Err.Number <> 0 Then MsgBox "There's been an error #1" Else MsgBox z End IF z = x * y If Err.Number <> 0 Then MsgBox "There's been an error #2" Else MsgBox z End If End Sub
The division by zero on the fifth line of the script in Example 4.8 generates an error. Therefore, the conditional
statement on line 6 evaluates to
True, and an
error dialog is displayed. Program flow then continues at line 12.
Line 12 is a perfectly valid assignment statement that always
executes without error, but the Err.Number property still contains
the error number from the previous error in line 5. As a result, the
conditional statement on line 13 evaluates to
True, and a second error dialog is displayed.
Despite the two error messages, though, there’s only been a
single error in the script.
The Err object can be reset by using the Clear method (which is discussed in the next Section 188.8.131.52).
The Description property contains a string that describes the last error that occurred. You can use the Description property to build your own message box alerting the user to an error, as the WSH script in Example 4.9 shows.
Example 4-9. Using the Description Property to Display Error Information
Dim x, y ,z On Error Resume Next x = 10 y = 0 z = x / y If Err.Number <> 0 Then MsgBox "Error number " & Err.Number & ", " & _ Err.Description & ", has occurred" Err.Clear Else MsgBox z End If z = x * y If Err.Number <> 0 Then MsgBox "Error No:" & Err.Number & " - " & _ Err.Description & " has occurred" Err.Clear Else Alert z End If
The Source property contains a string expression that indicates the class name of the object or application that generated the error. You can use the Source property to provide users with additional information about an error; in particular, about where an error occurred.
The value of the Source property for all errors generated within scripted code is simply “Microsoft VBScript runtime error.” This is true of all VBScript scripts, whether they’re written for Active Server Pages, Windows Script Host, Internet Explorer, or Outlook forms. Obviously, this makes the Source property less than useful in many cases. However, you can assign a value to the Source property in your own error handling routines to indicate the name of the function or procedure in which an error occurred. In addition, the primary use of the Source property is to signal an error that is generated by some other object, like an OLE automation server (like Microsoft Excel or Microsoft Word) or a COM component.
The Err. Raise method allows you to generate a runtime error. Its syntax is:
ErrorNumber is the numeric code for
the error you’d like to generate. At first glance, generating
an error within your script may seem like a very odd thing to want to
do! However, there are times, particularly when you are creating
large, complex scripts, that you need to test the effect a particular
error will have on your script. The easiest way to do this is to
generate the error using the Err.Raise method and providing the error
code to the
ErrorNumber parameter, then
sit back and note how your error handling routine copes with the
error, what the consequences of the error are, and what side effects
the error has, if any. The client-side script in Example 4.10, for instance, allows the user to enter a
number into a text box, which is passed as the error code value to
the Err.Raise method. If the value of the error code is nonzero, an
Alert box opens that displays the error code and its corresponding
description. Figure 4.6, for instance, shows the
Alert box that is displayed when the user enters a value of 13 into
the text box.
Example 4-10. Calling the Err.Raise Method
<HTML> <HEAD> <TITLE>Using the Err Object</TITLE> <SCRIPT LANGUAGE="vbscript"> Sub cmdButton1_OnClick On Error Resume Next errN = Document.frm1.errcode.value Err.Raise(errN) If Err.Number <> 0 Then Alert "Error No:" & Err.Number & " - " & Err.Description Err.Number = 0 End If End Sub </SCRIPT> </HEAD> <BODY BGCOLOR="white"> <CENTER> <H2>Generating an Error</H2> <P> <FORM NAME="frm1"> Enter an Error Code <INPUT TYPE="text" NAME="errcode"> <INPUT TYPE="button" NAME="cmdButton1" VALUE="Generate Error"> </CENTER> </BODY> </HTML>
At present there is no definitive list of VBScript runtime error codes available from Microsoft. Table 4.1 lists a few of the most common runtime errors.
An Error Code Generator
ERRCODES1.VBS), which allows you to generate a
complete list of current VBScript error codes, can be found on the
O’Reilly Visual Basic web site at
Table 4-1. Some Common VBScript Error Codes
Invalid procedure call
Out of memory
Subscript out of range
Division by zero
The Clear method clears the information
that the Err object is storing about the previous error; it takes no
parameters. It sets the values of
and the Err object’s Source and Description properties to
 A more complete version of the syntax of the Raise method is:
source is the name of the
module that generates the error, and
description is a string describing the
error. The latter parameter is useful in particular when handling an
application-defined error. This topic—and therefore the complete
syntax of the Raise method—is beyond the scope of this chapter.