Question

I’ve been reading a lot about this since I’ve been asked to fix a C# application that has memory leaking problems, but I haven’t found an answer for these 2 issues:

Consider the following code:

   private static ArrayList list = new ArrayList();

   public void Function()
   {
     list.add(object1);
     list.add(object2);

     //didn't call clear() prior to reusing list
     list = new ArrayList();
   }

Since the list wasn’t cleared before creating a new one, will this generate some sort of garbage that won’t be released after the static list itself is disposed?

The second issue is regarding Form.Dispose(). I see that a lot of controls available on designer view (i.e. labels, picture boxes) require disposing. It’s seems as though calling Dispose() on a Form causes all of these types of controls to be disposed also (correct me if I’m wrong), which is odd since the designer adds an overriden void Dispose(bool disposing) method which does no such thing. I’m assuming that this happens at the void Dispose(bool disposing) method of the base Form class.

The problem with the above is that it is not very clear to me what I need to do to ensure that all of the Form’s resources are disposed correctly. I do not understand how the Form knows which objects it needs to dispose. For example, if in my form I have a field which is a custom IDisposable object, will the Form know that it needs disposing? Or should I add the code necessary to release the object myself?

Also, if I do need to add the code to dispose certain objects, then how do I deal with the fact that the designer has already overriden the void Dispose(bool disposing) method? Should I edit the designer generated code or is there a cleaner way to do this?

I hope that this wasn’t to confusing, it’s a bit hard to explain. Thanks

Answer 1

No, that’s not a leak. When the garbage collector goes searching for object references, it won’t find a reference to the original ArrayList anymore. You replaced it. So it will automatically destroy that original ArrayList object, as well as all of its elements if they are not referenced anywhere either.

The Form class knows how to dispose itself, as well as all the controls that are child windows on that form. This happens when the user closes the form, the WM_CLOSE message that Windows sends triggers this code. The Form.Controls collection helps it find the reference to all child controls so it can dispose them too.

However, this will not happen if you remove controls from the form yourself. Now it is up to you to call Dispose() on them. Particularly the Controls.Clear() method is dangerous. What is unusual about it is that this causes a permanent leak, the controls you remove are kept alive by the ‘parking window’. Which keeps the window handle alive so you can move them elsewhere, on another container window for example. If you don’t actually move them, they’ll stay hosted on that parking window forever. No other classes in the framework quite behave this way.

This leak is easy to diagnose with Taskmgr.exe, Processes tab. View + Select Columns and tick USER Objects. If this steadily goes up while your program runs then you’re leaking controls.