Why Does Application.OpenForms.Count Always Return Zero in My Windows Forms App?

Application.OpenForms.Count Always Returns Zero

In Windows Forms applications, accessing the Application.OpenForms property is commonly used to obtain a collection of currently open forms. However, there is a known bug that can lead to the Application.OpenForms.Count property consistently returning zero.

This issue occurs when certain properties are modified after the form's creation. These properties include:

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

When any of these properties are changed after CreateWindowEx() has been called to create the form's window, Windows Forms must destroy the original window and create a new one with the updated style flags. However, due to a bug, the Application class doesn't correctly add the new form to the OpenForms collection. This results in the Application.OpenForms.Count property always returning zero.

Solution

To avoid this bug, always set any of the above-mentioned properties only in the form's constructor. Avoid changing them in event handlers or other code that executes after CreateWindowEx() has been called.

Alternatives to Application.OpenForms

In general, it's advisable to avoid relying on the Application.OpenForms property due to this bug. Instead, consider passing the form instance to the class that needs to display the message box through its constructor. MessageBox typically automatically determines the correct parent window and will display the message box over the active window in most cases.

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