The COM technology behind objects such as the Microsoft Scripting Runtime Library's FileSystemObject object (aka the FSO object) and Windows Script Host's (WSH's) WshShell object is what makes WSH scripts so powerful. However, the way this technology works causes scripters grief in one area: Scripts don't get access to the crucial constants embedded in objects.
If you use the FSO object's Open-TextFile method to open a file for writing or appending, you might have run into this problem. In its simplest form, this method takes one mandatory parameter—the name of a file—and opens that file so you can read it. For example, the code
Dim fso
Set fso = CreateObject( _
"Scripting.FileSystemObject")
Set file = fso.OpenTextFile( _
"C:\temp\computers.txt")
opens C:\temp\computers.txt for reading. The OpenTextFile method has three optional parameters, one of which is the I/O mode. When you look at the documentation for using this parameter, you'll notice that it can take one of three constants: ForReading, ForWriting, or ForAppending. However, if you try to use ForAppending to open and add data to the file with code such as
Set file = fso.OpenTextFile( _
"C:\temp\computers.txt", _
ForAppending, True)
VBScript will immediately give you the runtime error Invalid procedure call or argument. If you're a cautious scripter who uses Option Explicit to make sure all variables are defined, you get an even more puzzling but more accurate error message: Variable is undefined: 'ForAppending'. The problem is that you're using ForAppending as a constant, but the VBScript engine sees it as a variable. You'll receive similar error messages when you use ForReading and ForWriting. . . .
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