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Programming Interfaces
External Program Interfaces : Using COM Objects in ABL : Managing COM objects in an application : Managing COM object resources : Releases and component handles
Releases and component handles
When you release a COM object, this invalidates every component handle that references the same object. Any further attempt to use an invalidated component handle results in an error indicating that an invalid action was performed on an invalid COM-HANDLE value.
On the other hand, if you instantiate a different COM object after releasing the first one, a previously invalidated component handle might point to the new COM object. The component handle might be reused because the object server might reuse memory left over from the released COM object. In this case, the component handle might be seen as valid. However, the object that the component handle references is different, so this might result in errors or successful method calls with results that are invalid for your application.
In general, to avoid errors and confusion with invalidated COM handles, it is a good practice to set any COM-HANDLE variables to the Unknown value (?) once you have released it.
Note: ABL maintains no relationship between one component handle and another, such as when you derive one component handle from a property value on another component handle. The two component handles are completely independent of each other. This is different from the parent/child relationships that some COM objects (especially Automation objects) might maintain among themselves. In this case, invoking the right method on a "parent" COM object might well trigger the release of many other "related" COM objects. For information on any such cascading object relationships, see the documentation on a particular Automation Server or ActiveX control.