It appears as MDI Form is keeping EventHandleList around even after an MDI child form closes. This leaks objects associated with event handlers.
I have used DevPartner to track those down. One place that reliably creates the leak is line #1539 in Renderers.cs, where toolTip.SetToolTip(..,..) associates tool tip text with control.
The memory leak occurs only if the parent from is an MDI form.
Another interesting aspect is that if you create a delegate on UI thread the _target seems to be leaking as if it’s kept in an EventHandlerList.
I''ve found this article online that confirms my findings: