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Using the CreateObject and GetObject Functions to Instantiate Objects

Because many COM components do not yet support the As New keyword, you will need to create and instantiate object variables for these components' classes with standard variable declarations and the CreateObject or GetObject functions.

The CreateObject and GetObject functions return a reference to a server class object. Use them with the Set keyword to assign their return values to a previously declared class object variable:

Set objExcel = GetObject(,"Excel.Application")

or

Set objExcel = CreateObject("Excel.Application")

The CreateObject function takes a single required argument, which is the name of the class you're instantiating. It always instantiates a new object in your application. It also takes a second optional argument which is a string representing the share name of the server where you can create a remote object.

GetObject

You can use GetObject to create an object from an already running instance of a server.

GetObject takes two possible parameters. You must always specify at least one of the two parameters:

  • GetObject's first parameter is a String giving the path and filename of a data file associated with the server application and its class.

  • GetObject's second parameter is the same as CreateObject's single parameter: the name of the class you're instantiating.

There are several rules to keep in mind when using GetObject's parameters:

  • If you leave the first parameter completely blank (that is a single comma before the second argument), GetObject will always reference an existing object. If there is no existing object, a runtime error occurs.

  • If you specify a valid filename in the first parameter and the file is of the type associated with the server application, you may leave the second parameter blank. GetObject will open the file with the associated server application. GetObject will use an existing reference to the object if it exists, or it will open a new copy of the object if none existed before in the application.

  • If you specify a blank filename (" ") in GetObject's first parameter, then you must specify the second parameter. GetObject will then always open a new copy of the object regardless of whether one already exists in the application.

The possible configurations of these two parameters are summarized in Table 10.1.

TABLE 10.1POSSIBLE COMBINATIONS OF FILENAME AND SERVER.CLASS ARGUMENT SETTINGS FOR GETOBJECT FUNCTION

FileName

Server.Class

Effect
Blank Blank Not a possible combination.
Blank Server.Class Always uses an existing instance.

Runtime error if there is no existing instance.
Empty String Server.Class Always opens a new instance of the server.
FileNameBlank Opens server of type associated with FileName.

Uses instance if Available otherwise opens new
instance.
FileNameServer.Classfile specified with a new instance of the server.

IMPORTANT - Limitations on Support for GetObject: The following discussion of GetObject applies to many Microsoft products and other COM components. However, the use of GetObject is applicationspecific. In fact, some ActiveX server applications don't support GetObject at all. You should refer to the application's documentation to find out whether GetObject is supported and, if so, what the proper syntax would be for using GetObject. If GetObject isn't available, you must always use CreateObject

Comparing GetObject and CreateObject

Since several exam questions usually rely on the confusion between GetObject and CreateObject, it is useful to emphasize the difference between these two functions:

  • CreateObject always creates a new instance of the server object.

  • GetObject can use a running instance of the server object but can also create a new instance depending on the syntax you use, as detailed in the following section.


  

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