Memory leaks in a Windows Forms application

Question

We are developing a big .NET Windows Forms application. We are facing a memory leak/usage problem in that despite we are disposing the forms.

The scenario is like:

1. Our application is using 60 KB of memory with a list of records displaying in a grid.
2. When the user clicks on a record it opens a form, myform.showDialog, show the details. The memory jumps from 60 KB to 105 MB.
3. Now we close the form myform to get back to grid, and dispose that form and set it to null. Memory remains at 105 MB.
4. Now if we again perform step 2, it would jump from 105 MB to 150 MB and so on.

How can we free up the memory when we close myForm?

We have already tried GC.Collect(), etc., but without any result.

Answer 1

First, check for references to your form. Does your form subscribe to any events? Those count as references, and if the event publisher lives longer than your form, then it will keep your form around (unless you unsubscribe).

It could also be a coincidence -- .NET allocates memory in segments, I believe, so you may not see your working set go down with every form release (the memory is released by your form, but is still held for the next allocation by your application). Since your memory allocation is, at least, one abstraction away, you won't always get behavior where your working set goes up and down by the exact number of bytes you allocate.

A way to test this is to create a lot of instances of your form and release them -- try to magnify the leak so that you allocating and releasing 100's of instances. Does your memory continue to step up without going down (if so, you have a problem), or does it eventually return to close to normal? (probably not a problem).

Answer 2

The first place to look for leaks is in event-handling rather than missing Dispose() calls. Say your container (the parent form) loads a child form and adds a handler for an event of that child form (ChildForm.CloseMe).

If the child form is intended to be cleared from memory then this event handler must be removed before it is a candidate for garbage collection.