Unloading Objects from Memory

Microsoft® Windows® 2000 Scripting Guide

In-process servers (that is, Automation objects encapsulated in .dll files) will automatically unload themselves from memory when the calling script completes. This is because these objects run in the same process as the script; when the script process ends and is thus removed from memory, any in-process servers will also be stopped and removed from memory. For example, the following script creates an instance of the FileSystemObject and then displays a message box. As soon as you dismiss the message box, both the script and the FileSystemObject are removed from memory.

Set TestObject = CreateObject("Scripting.FileSystemObject")
Wscript.Echo "Click here to end the script."

This is not true, however, for out-of-process servers, Automation objects that run in a different process than the script itself. For example, the following script creates an instance of Microsoft Word and then displays a message box. When you dismiss the message box, the script process is unloaded from memory.

Set TestObject = CreateObject("Word.Application")
Wscript.Echo "Click here to end the script."

However, the Microsoft Word process (Winword.exe) will continue to run and remain in memory, even though it is not visible on the screen. This is because there is no inherent tie between the script process and the Word process; anything you do to the script process does not affect the Word process and vice versa. You can verify that the process is still running and verify the amount of memory it is still allocated by using Task Manager, as shown in Figure 2.29.

Figure 2.29 Automation Object Running After a Script Has Completed

With out-of-process servers, you will typically have to use the method built into the object to explicitly unload it from memory. (You will need to check the documentation for the object to determine that method.) Microsoft Word, for example, is unloaded from memory by using the Quit method. The following script creates an instance of Microsoft Word and then immediately unloads that instance using the Quit method.

Set TestObject = CreateObject("Word.Application")
TestObject.Quit

If you run the preceding script and then check the processes running on the computer, you will not see Winword.exe (unless, of course, you had multiple copies of Winword.exe running).

Nothing Keyword

VBscript includes the Nothing keyword, which can be used to disassociate an object reference and an object. After an object variable is set to Nothing, the variable no longer maintains an object reference and thus cannot be used to control the object. For example, the following code creates an instance of Microsoft Word, sets the object variable to TestObject, and then tries to use TestObject to quit Word and unload the object from memory.

Set TestObject = CreateObject("Word.Application")
Set TestObject = Nothing
TestObject.Quit

When this script runs, the error message shown in Figure 2.30 appears. The script fails because TestObject no longer represents a valid reference.

Figure 2.30 Working with an Invalid Object Reference

Setting an object variable to Nothing releases a small amount of memory but does not unload the object itself from memory. Because of that, there is generally no reason to set an object variable to Nothing; in effect, object variables (and all other variables, for that matter) are set to Nothing when the script completes. For example, in the following script the last line of code is superfluous: It sets the object variable TestVariable to Nothing, but that would occur anyway as soon as the script ended.

Set TestObject = CreateObject("Scripting.FileSystemObject")
Set TestObject = Nothing