Control._reflectParent is not updated when Parent changes (memory leak)

.NET version

.NET 7.0.5 (latest public release)

Did it work in .NET Framework?

No

Did it work in any of the earlier releases of .NET Core or .NET 5+?

I don’t think this is a regression, it was probably always like this. I’ve never needed to worry about this in the past – my use of Controls did not include reparenting until now.

Issue description

The Control._reflectParent field points to a control’s parent control. It is updated by calling set_ReflectParent via UpdateReflectParent(), which is called by CreateHandle(), Dispose(bool), and OnHandleDestroyed(EventArgs).

Notably absent from that list is OnParentChanged() (or any method to that effect). This means that _reflectParent is not set to null when a control is removed from its parent, nor is it updated when the control is added to a new parent.

Here is the ILSpy analysis tree for _reflectParent :

This results in a memory leak if a control is re-used, which is something I’m doing in Paint.NET to implement a large memory use optimization. This optimization depends on a control’s Handle not being recreated during a reparenting operation (which does work fine).

In SciTech Memory Profile we can see this clearly. In this case, the CanvasControlImpl (child) has been removed from the CanvasControl (parent), but it (child) still has a reference to it (parent) whether it (child) has been reparented yet or not. The yellow exclamation mark indicates that CanvasControl is disposed.

I don’t know if there are other behavioral bugs that result from this – ReflectParent only seems to be read by OnParentHandleRecreated().

Steps to reproduce

1. Create two Controls, c1 and c2
2. Add c1 to a Form
3. Add c2 as a child of c1
4. Show the form
5. Remove c2 from c1
6. Inspect c2._reflectParent. It should be null, but it still references c1.

[JeremyKuhne]

@Nora-Zhou01 there really isn’t much of value you can do to repro- I added a unit test that does this.

Apparently, I didn’t mark my PR correctly. I’m closing this now.

[rickbrew]

@rickbrew We have made some attempts but we don’t seem to find the most accurate sample app, could you please provide the sample project, thanks in advance.

I’m not sure I understand … @JeremyKuhne landed a fix for this in #9159. What is it exactly that you need from me? Like a minimal repro? As in, a Visual Studio project you can run that will reproduce the issue?