Avoiding Showing or Opening More than One instance and Showing the Current One

It seems you should implement a mechanism to distribute your form to all the classes how should be able to show it. If your distribute the reference pointer, you will always get the same instance of the form.

So say you have a Form. You can create a instance of that in your main class. Then you pass it down to your sub-classes either in the constructor for simple apps, or as a property (FYI: you actually will then just copy the reference pointer to that form). You then use Show and Hide. It is now important that you override the closing of the form by handling the Closing event and setting the Cancel property of the CancelEventArgs to true, and calling Hide instead.

You can use the singleton design pattern[^] for that. That pattern ensures that there's only one instance of a class.

example code for a WPF Window:

public sealed partial class SingleWindow : Window {
    private static SingleWindow instance;
    private bool isMainExit=false;
    private SingleWindow() {
        InitializeComponent();
        App.Current.MainWindow.Closed+=new EventHandler(MainWindow_Closed);
    }
    public static SingleWindow Instance {
        get {
            if (instance == null) {
                instance = new SingleWindow();
            }
            return instance;
        }
    }
    protected override void OnClosing(System.ComponentModel.CancelEventArgs e) {
        if (!isMainExit) {
            this.Hide();
            e.Cancel = true;
        }
        base.OnClosing(e);
    }
    private void MainWindow_Closed(object sender, EventArgs e) {
        this.isMainExit=true;
        this.Close();
    }
}

After you close a form/window that form/window enters a state where you can't use it anymore (until the garbage collecter removes it entirely, I suppose). Therefore you need to check wether the form/window has been closed. Forms get disposed so you check for IsDisposed. I couldn't immediately find a similar mechanism for a WPF Window so I implemented a workaround. If anybody knows a more elegant solution, please correct me. :-)

edit: replaced WinForms code by WPF code