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up vote 3 down vote favorite

I've read (on using Statement (C# Reference)) that the using statement should be used to release the resources used by managed types (like File and Font) that use unmanaged resources. So started using it with MySql classes and related stuff, but if you take a look at an object of a Windows.Forms.Form class you will see a Dispose method, it means that this class implements IDisposable so, should I use a using statement for a Windows.Forms.Form object like in the case below?

private void aboutToolStripMenuItem_Click(object sender, EventArgs e)
{
    using (AboutBoxProjeto about = new AboutBoxProjeto())
    {
        about.ShowDialog();
    }
}
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3  
Yes, but only around ShowDialog(). It won't work with Show(). –  Henk Holterman May 17 '13 at 13:26
    
Definitely. Failure to dispose a Form displayed with ShowDialog is a memory leak. –  Ginosaji May 17 '13 at 13:31
    
@Ginosaji that is not correct. The finalizer prevents that. It is a safety net that exists in order to avoid a resource leak if the programmer omits disposing an instance. –  user1222021 May 17 '13 at 13:56
    
@Ginosaji - seems I'm wrong. From the documentation supplied by the accepted answer, It seems that modal forms need to be disposed after all. I suppose there is a hidden mechanism that will keep a live reference to it that will prevent the Finalizer to run. –  user1222021 May 17 '13 at 14:02
    
@Ginosaji - sorry, I was right - I built a test project and could verify that a form opened with ShowDialog() is still properly disposed by the finalizer if we omit disposing it explicity. =) –  user1222021 May 17 '13 at 14:23

3 Answers 3

up vote 5 down vote accepted

From http://dotnetfacts.blogspot.com/2008/03/things-you-must-dispose.html:

In .NET, a dialog form is a form opened by calling the ShowDialog() method. Unlike modeless forms, the Close method is not called by the .NET Framework when the user clicks the close form button of a dialog box or sets the value of the DialogResult property. Instead the form is hidden and can be shown again without creating a new instance of the dialog box. Because a form displayed as a dialog box is not closed, you must call the Dispose() method of the form when the form is no longer needed by your application

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What about Windows.Forms.Form objects open with the Show() method? They should be disposed too? –  Zignd May 17 '13 at 13:33
1  
As mentioned in quote, if form is modeless (that means opened with Show()), on closing form dispose is performed by framework. –  Giedrius May 17 '13 at 13:34
    
Thanks for you answer Giedrius. :) –  Zignd May 17 '13 at 13:36
up vote 0 down vote

That is correct. As a rule of thumb, if a class implements the IDisposable interface, it is a best-practice to dispose it whenever an instance is no longer needed. This will release resources in an early, deterministic way. If you fail to do so, the finalizer mechanism will act as a safety net and the resources will still be released, however, you don't have control when this will actually happen.

Edit: After reading the links given by the accepted answer, it seems that a call to 

Dispose()

IS required when the form is shown modally. I suppose there is an internal mechanism holding references to the form that prevents the finalizer to run at all.

Edit #2: No, after building a simple test project, I could validate that my initial assumption was correct. The finalizer safety net is able to run and will properly release a modal form's resources (opened with ShowDialog()) - in an undeterministic way.

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up vote 0 down vote

Yes, for short-lived forms (such as dialog boxes) I would definitely recommend wrapping them in a using block as you do in your code sample, just to make sure they are properly released.

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