Why Does Application.OpenForms.Count Sometimes Return 0 in Windows Forms?

Understanding Application.OpenForms.Count: A Common Misconception

In Windows Forms applications, accessing the Application.OpenForms collection is frequently used to obtain the currently open forms. However, situations may arise where the collection consistently returns a count of 0, leading to confusion.

The True Nature of the Issue

Contrary to popular belief, the Application.OpenForms collection does return accurate results in most cases. It stores a list of currently open forms, providing a snapshot of the application's state. However, a specific bug in Windows Forms can cause certain form properties to become inaccessible after initialization.

When properties like ShowInTaskbar, FormBorderStyle, ControlBox, or Opacity are manipulated post-initialization, Windows Forms is unable to modify the native window's style flags effectively. This leads to the original window being destroyed and a new one being created. While the new window adopts the modified property settings, the Application class is unaware of its creation. As a result, the Application.OpenForms collection remains outdated, showing a count of 0 for the form in question.

Avoiding the Pitfalls

To mitigate this bug, it's crucial to set these affected properties solely in the form constructor, before CreateWindowEx() is invoked. This ensures that Windows Forms can correctly update the window's style flags without causing the undesirable flickering and miscounting issue.

Alternative Approaches

Due to the potential for the OpenForms collection to return incorrect results, it's recommended to explore alternative methods for obtaining the current form:

• Pass the form reference to the class that needs to display the message box through its constructor.
• Utilize MessageBox's ability to automatically determine the active window and use that as the parent.
• For cross-platform scenarios, consider implementing a custom method to retrieve the current form, using SynchronizationContext.Current to properly manage thread affinity.

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