Why Does `Application.OpenForms.Count` Return 0 After Modifying Certain Form Properties?

Application.OpenForms.Count Conundrum with Form Properties

Ever wondered why Application.OpenForms.Count always returns 0? This perplexing issue arises under specific circumstances when modifying certain form properties after its creation.

Understanding the Bug

Windows Forms has a known bug that causes a form to vanish from the Application.OpenForms collection if its ShowInTaskbar, FormBorderStyle, ControlBox, Min/MaximizedBox, RightToLeftLayout, HelpButton, Opacity, TransparencyKey, ShowIcon, or MdiParent properties are modified post-creation. These properties set style flags in the native CreateWindowEx() call, requiring the system to recreate the window with the updated style.

When the original window is destroyed, Application.OpenForms loses track of it. However, it fails to read it back when Windows creates the new window.

Avoiding the Bug

To avoid this bug, set the aforementioned properties exclusively in the form's constructor, before Windows calls CreateWindowEx(). Avoid modifying these properties in event handlers or any code that executes after window creation.

Alternatives to Application.OpenForms

Given the potential for issues with OpenForms, it's advisable to adopt alternative approaches. Consider providing the message box's owning form directly as a parameter in the displaying class's constructor. In most cases, the MessageBox can automatically determine the appropriate parent window.

If invoking message boxes from a worker thread, ensure the current SynchronizationContext is passed in the constructor and used for invoking Post(). This approach ensures compatibility with other GUI frameworks.

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