Hybrid objects assigned to System.Windows.Forms.Timer:Tag are never garbage collected

Hybrid objects assigned to System.Windows.Forms.Timer:Tag are never garbage collected

When assigning a hybrid object (ABL object inheriting from a .Net object) to the Tag property of a System.Windows.Forms.Timer and removing it again in the Timer:Tick event, the object never gets garbage collected.  For example the hybrid class is:

USING System.Object FROM ASSEMBLY.
BLOCK-LEVEL ON ERROR UNDO, THROW.

CLASS SimpleObject
    INHERITS Object:
END CLASS.

If this is assigned to a timer Tag property:

DEFINE PRIVATE VARIABLE Timer AS System.Windows.Forms.Timer NO-UNDO.
...
THIS-OBJECT:Timer = NEW System.Windows.Forms.Timer(THIS-OBJECT:components).
...
Timer:Tag = NEW SimpleObject().
Timer:Start().

The timer Tick event handler removes the reference to the hybrid class, so it should be garbage collected:

METHOD PRIVATE VOID Timer_Tick(sender AS Object, e AS EventArgs):
        Timer:Stop().
        ASSIGN Timer:Tag = ?.
END METHOD. /* Timer_Tick */

But this never happens.  With DynObjects.Class client logging it can be seen in the log that the object is never garbage collected:
 

If the hybrid object is replaced by a pure .Net object, the problem doesn't happen.  For example:

DEFINE PRIVATE VARIABLE Timer AS System.Windows.Forms.Timer NO-UNDO.
...
THIS-OBJECT:Timer = NEW System.Windows.Forms.Timer(THIS-OBJECT:components).
...
Timer:Tag = NEW System.Object().
Timer:Start().

 

This is expected behavior. 

The objects are being garbage collected, but this isn't happening immediately because .NET simply hasn't performed garbage collection yet.  In addition to this the OpenEdge client waits until .Net has completed garbage collection before it garbage collects the ABL side of the objects.  In this case when .Net does perform garbage collection the client log has already been closed.  If the code forces garbage collection in the Timer_Tick routine of TimerLeakForm:
 

System.GC:Collect().
System.GC:WaitForPendingFinalizers().
System.GC:Collect().

Then this will force the objects to be garbage collected and shows the ABL side of the objects being garbage collected in the client log:
The fact that these objects appear to be left is something peculiar to hybrid objects and is explained in article  Objects not deleted from memory immediately.  It's not possible for the OpenEdge client to know when .Net will perform garbage collection and it shouldn't be necessary (and isn't recommended) to force it as described above. But Microsoft explain the algorithm they use at:

https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/fundamentals
 

Progress Article(s):
 Objects not deleted from memory immediately