Memory leak when installing event handlers

Environment

• Pythonnet version: 2.5.2
• Python version: 3.8.3
• Operating System: Windows 10 Home (Build 19041.804)
• .NET Runtime: 4.8.04084 (I think?)

Details

• Describe what you were trying to get done.

Develop a native WinForms application.

• What commands did you run to trigger this issue?

Any WinForms app that has an event handler should exhibit the problem. Here’s a minimal example derived from the HelloApp example in the pythonnet codebase.

import clr

SWF = clr.AddReference("System.Windows.Forms")
print (SWF.Location)
import System.Windows.Forms as WinForms
from System.Drawing import Size, Point


class HelloApp(WinForms.Form):
    def __init__(self):
        self.ClientSize = Size(400, 200)

        self.button = WinForms.Button()
        self.button.Location = Point(50, 50)
        self.button.Size = Size(300, 20)
        self.button.Text = "Click Me!"

        self.button.Click += self.button_Click

        self.Controls.Add(self.button)

    def button_Click(self, sender, args):
        print ("Click")
    
    def run(self):
        WinForms.Application.Run(self)

def main():
    form = HelloApp()
    app = WinForms.Application
    app.Run(form)


if __name__ == '__main__':
    main()

Run the app, then really mash the “click me” button (lots - like, dozens of times). Every time you click, the app’s memory allocation increases a little bit; I’ve never seen this allocation be freed by a garbage collection cycle, even if you add a gc.collect() call.

This problem was observed in practice with code that integrates the Python Windows Proactor with the Winforms event loop. In that code, we’re creating and scheduling a System.Action() every 5ms to schedule an iteration of the asyncio event loop - and every time it does, there’s a small leak. Invoke that 200 times a second, and the leak adds up quickly. However, even simple button presses cause a leak.

• If there was a crash, please include the traceback here.

No crash - the app works fine; it just (slowly) leaks memory.

[freakboy3742]

I’ll add that if this is a situation where money will accelerate progress, I may be in a position to help out.

[freakboy3742]

@filmor I’ve just tested the current master (23527d11) - I’m not seeing any change in behavior.