Public Class myForm
    Inherits Form

    Protected Overrides Sub OnClosed(ByVal e As EventArgs)
        MessageBox.Show("The form is now closing.", "Close Warning", _
            MessageBoxButtons.OK, MessageBoxIcon.Warning)
        MyBase.OnClosed(e)
    End Sub

    Public Sub New()
        MyBase.New()
    End Sub

End Class

public class myForm:
    Form

{
    protected override void OnClosed(EventArgs e)
    {
        MessageBox.Show("The form is now closing.", 
            "Close Warning", MessageBoxButtons.OK, MessageBoxIcon.Warning);
        base.OnClosed(e);
    }

    public myForm() : base()
    {        
    }

}

public ref class myForm: public Form
{
protected:
   virtual void OnClosed( EventArgs^ e ) override
   {
      MessageBox::Show( "The form is now closing.", "Close Warning", MessageBoxButtons::OK, MessageBoxIcon::Warning );
      Form::OnClosed( e );
   }

public:
   myForm()
      : Form()
   {}

};