How do I know if an object will be disposed automatically when i invoke the close() method on the form?

Question

Okay, I have a problem with my program climbing in memory usage until it crashes the computer. I am almost completely sure that this is because one or more objects are not being disposed of from when i open and close secondary forms. How can I tell which variables arent being disposed of?

Vulpes · Answer

@Patrick, I believe there's a built in profiler in the premium editions and above (which I don't have) which can probably show this information. @ Sam Yes, it is referring to resources at the Windows API level - in fact, when MSDN talks about 'resources' I always assume they mean unmanaged resources unless the context implies otherwise. When Form.Close() is called and the form handle has been created, the WM_CLOSE message is sent to the form's window procedure which triggers calls to DestroyWindow on both the child windows and the form window itself. The managed wrappers for the Form and its controls will continue to exist (albeit in an unstable state) until there is no longer a reference to the Form object and the GC cleans them up. In the second paragraph, both conditions have to apply for the form not to be disposed of when Close() is called which, AFAIK, is correct. However, I certainly agree with your general thrust that MSDN is often misleading and, indeed, sometimes plain wrong!

Suthish Nair · Answer

If the query got resolved, then mark the post that helped you as Answer so will be helpful for others. Otherwise if you got the solution please share with us.

Sam Hobbs · Answer

Vulpes: I am not an expert but I think your quote from the MSDN is misleading. It is surprising that it says that when "a form is closed, all resources created within the object are closed and the form is disposed". I just don't believe it. It is totally possible to allocate a form, then set a member of the form to something, then show the form, and then the form changes the value, then the form closes and the object that created the form can then get the modified value. It would be a big problem if all members of the form are disposed of when the form is closed. Probably the key word here is " resources " which is not synonymous with members. It is probably referring to resources at the Windows API level. Also, an MDI child form is never a startup form ! So saying that a form is not disposed of when it is closed when it is part of a MDI application sounds to me to also be misleading, but since it says " and " shown using ShowDialog, it is probably true and must be read carefully, as is the case for many things in the MSDN.

Sam Hobbs · Answer

Garbage collection is supposed to make programming easy, but I think you are currently frustrated by it. There are many useful articles in this web site. Have a look at the following. If they don't help then you need to look for similar articles in the MSDN and other sites. .NET Best Practice No: 1:- Detecting High Memory consuming functions in .NET code Best Practices No 5: - Detecting .NET application memory leaks Garbage Collection: Memory Management in .Net The "Using" Statement in C#

patrick · Answer

Let me ask this to make it easier. Does visual studio have a way for me to see all objects currently in memory?

Vulpes · Answer

Incidentally, are you using any large arrays, lists, dictionaries or other collections in your application? By 'large' I mean needing a memory allocation of more than 85K bytes which would inlcude, for instance, an array of 22,000 ints or 11,000 doubles. The problem with these 'large' objects is that they are allocated in a special area of the heap. Although this is GC'd (as part of generation 2) when large objects need destroying, active objects higher up in memory are not compacted down to fill up the space left because of the performance cost of moving large amounts of memory. This means that 'holes' may eventually appear in the LOH if new objects are too big to fit into an existing free slot and this can lead to you running out of memory even when, in reality, there's plenty left! Rather than repeatedly creating new large arrays, what I try to do is to estimate how big the largest array I'll need will be and then reuse this rather than creating new arrays. I also try to avoid not having too much stuff in memory at one time when I can easily swap it out from disk without too much of a performance penalty.

Vulpes · Answer

As a general rule, a class only needs to implement the IDisposable interface (and it's only method Dispose()) when it uses 'unmanaged' resources such as file handles, database connections etc. So, a Person class wouldn't normally need to do this. If a form is hidden but some code on it is running in the background, then the GC will perform normally. So, if there are no longer any references to a particular object, it will be disposed of in due course.

patrick · Answer

Let me ask this: do I need to manually add a dispose method to any classes that i create? A simple one like public class Person for instance? Also, if I have a form that is hidden, but running functions in the background, are any classes being disposed of when those specific functions finish being disposed of if this form never closes?

Vulpes · Answer

Perhaps I could refer you to this extract from the MSDN documentation for the Form.Close() method: "When a form is closed, all resources created within the object are closed and the form is disposed. You can prevent the closing of a form at run time by handling the Closing event and setting the Cancel property of the CancelEventArgs passed as a parameter to your event handler. If the form you are closing is the startup form of your application, your application ends. The two conditions when a form is not disposed on Close is when (1) it is part of a multiple-document interface (MDI) application, and the form is not visible; and (2) you have displayed the form using ShowDialog. In these cases, you will need to call Dispose manually to mark all of the form's controls for garbage collection."