ControlClosing.cs

    protected override void OnParentChanged(EventArgs e)
    {
        base.OnParentChanged(e);

        if (parentForm != null)
        {
            parentForm.Closing -= parentForm_Closing;
        }
        parentForm = FindForm();

        if (parentForm != null)
            parentForm.Closing += parentForm_Closing;
    }

    void parentForm_Closing(object sender, System.ComponentModel.CancelEventArgs e)
    {
        parentForm.Closing -= parentForm_Closing;
        parentForm = null;
        //closing code
    }