Understanding VBScript Errors

A syntax error is an error that occurs as a result of not properly formatting statements within scripts.

Errors may also be generated during script execution. These types of errors are referred to as run-time errors . Run-time errors only appear when the statements that generate them are executed. As a result, some run-time errors may not be detected when the script executes (if the statement containing the error is not executed). For example, a run-time error may be hidden deep within a function or subroutine that is seldom called.

A run-time error is an error that occurs when a script tries to perform an illegal action, such as multiplying a numeric and a character value.

With proper testing of all the components of a script, most run-time errors can be discovered and fixed during script development. I say "most" because not all runtime errors can be caught—those caused by unforeseen circumstances may be impossible to detect during script development. Perhaps the person running the script incorrectly supplied input in a manner that you could not have anticipated, or perhaps something is wrong with the environment in which the script is being executed. Or maybe the hard disk has become full, preventing your VBScript from writing to a file, or the network goes down as your script is executing a file copy or move operation. In cases such as these, often the best you can do is to end the script gracefully, without confusing the user or making the situation worse .

A logical error is an error produced as a result of a programming mistake on the part of the script developer.

Another category of error to which scripts are susceptible is logical errors. Logical errors are mistakes made by the script developer. For example, instead of looping ten times, you might accidentally set up an endless loop. Another example of a logical error is a situation in which a script adds two numbers that should have been multiplied.

Understanding Error Messages

VBScript error messages are generated for both syntax and run-time errors. These errors are displayed in the form of popup dialogs, as demonstrated in Figure 8.6. Each error message displays information about the error, including a brief description of the error, an error number, and the source of the error.

Figure 8.6: A typical VBScript error message

The error message displayed in Figure 8.6 is the result of a syntax error. As you can see, a lot of useful information about the error is automatically provided. The ability to interpret and understand this information is critical for trouble-shooting and fixing your VBScripts.

The following information has been provided in this error message:

• Script. The name and location of the VBScript that produced the error.

• Line. The line number within the VBScript where the error was detected.

• Char. The column number position within the line where the error was detected.

• Error. A brief description of the error.

• Code. An error number identifying the type of error.

• Source. The resource that reported the error.

You can see in Figure 8.6 that a VBScript named X.VBS located in C:\Temp generated the error. Line 6 of the script that generated this error looks like the following statement:

If X > 5 MsgBox "Hello World!"

This If statement uses the VBScript MsgBox() function to display a text string. The error message indicates that the problem is that VBScript expected to find the 'Then' keyword and did not. If you take a look at the middle of this statement you'll see that, in fact, the Then keyword is absent. To correct this error, you would add the missing keyword, like this:

If X > 5 Then MsgBox "Hello World!"

To verify that the error has been eliminated, you could then save and run the script again.

Fixing Syntax Errors

VBScripts are subject to many different types of syntax errors, which Microsoft documents as part of the VBScript documentation available at http://www.msdn.microsoft.com/scripting. For your convenience, I've provided this information in Table 8.1. In addition, I listed both the decimal and hexadecimal error number associated with each error.

Catching Run-Time Errors

VBScripts are also subject to a wide range of possible run-time errors. Microsoft documents these errors as part of their VBScript documentation, which is available at http://msdn.microsoft.com/scripting. For your convenience, I've provided this information in Table 8.2. In addition, I listed both the decimal and hexadecimal error numbers for each run-time error.