Why Does `Application.OpenForms.Count` Sometimes Return 0, Even When Forms Are Open?

Why Does Application.OpenForms.Count Always Return 0?

In certain scenarios, the Application.OpenForms.Count property may unexpectedly display 0 forms, even when multiple forms are open. This issue stems from a known bug within Windows Forms when specific properties are modified after a form's creation.

The properties that trigger this bug include:

• ShowInTaskbar
• FormBorderStyle
• ControlBox
• Min/MaximizedBox
• RightToLeftLayout
• HelpButton
• Opacity
• TransparencyKey
• ShowIcon
• MdiParent

When these properties are changed after window creation, Windows Forms attempts to recreate the window with the updated style settings but fails to include the form in the Application.OpenForms collection again. This inconsistency leads to an inaccurate count.

Resolving the Issue

To avoid this bug, ensure that the aforementioned properties are set solely within the constructor, before the CreateWindowEx() call is made. Altering these properties within event handlers or after window creation can result in the OpenForms issue.

Alternative Approaches

Instead of relying on Application.OpenForms, consider these alternative methods:

• Pass the form owner as a parameter to the MessageBox.Show() function directly.
• Have the class that needs to display the message box receive the form instance through its constructor.
• Utilize MessageBox's ability to determine the active window as the parent, which is accurate in most cases.
• For worker thread communication, copy SynchronizationContext.Current in the constructor and invoke Post() later to ensure compatibility with other GUI class libraries.

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