Why GUIDs Change On Every Recompile Of OLE Automation Server

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Consider two Visual Basic application instances, SRV and CTRLR. SRV is an OLE Automation Server and CTRLR is an OLE Automation Controller that has a reference to an object in SRV. If SRV shuts down and a major change (such as changing the name of a procedure or class, changing the number or type of the parameters, or removing a method) is made to it, CTRLR will still have a reference to the old type library and will bind to the old definition of the class. Thus CTRLR will now have an invalid reference to SRV. GUIDs always change on recompilation, in order to avoid such a scenario.

In object applications, changes you make to existing procedures may cause the new version to be incompatible with previous versions. For example, you could change either the name of a procedure, the number of arguments passed to a procedure, or the data type of arguments passed to a procedure.

For code that uses previous versions of the object application, such changes may cause errors. Visual Basic provides a way to define a version of your application as a reference, to which your changes are compared. This reference version is called the "compatible object application." If a change is made that might cause incompatibility with the compatible object application, Visual Basic displays a warning message to alert you to the possible incompatibility.

Therefore, to work around the behavior:

1. When you begin a project, decide on the names of all the classes that will be exposed while making the project.
2. Add a .CLS module for each class. Make it public and creatable, and set the name for the class.
3. Make an .EXE file.
4. Set the .EXE file as the "compatible object application." To specify a compatible object application:
   
   
   1. From the Tools menu, choose Project Options. The Project Options dialog box appears.
   2. In the Compatible Object Application field, enter the full path and filename of the .EXE file you want to use as the basis for identifying possible version incompatibilities.
   3. Choose OK.
5. Be sure to update the compatible server when a class name is changed, a class attribute is set, or a new class is added.

If an incompatible change is made to the object, and you go ahead and recompile the server application despite the warning, you will have to close the client application and re-start it in order for it to read the system registry again to get the new GUIDs for the object.

Even if you make a compatible change, such as adding a new method, there is no mechanism to tell the running instances of the controllers to use the new version of the interface.