关闭所有子窗体

FormCollection form = Application.OpenForms;
            ArrayList slist = new ArrayList();
            IEnumerator form1 = form.GetEnumerator();
            while (form1.MoveNext()) {
                Form ff = (Form)form1.Current;
                if(ff!=this)
                slist.Add(ff);
            }
            IEnumerator s = slist.GetEnumerator();
            while (s.MoveNext())
            {
                Form f = (Form)s.Current;
                f.Close();
            }
为什么我这个代码把主窗体的也关闭了(它是在执行完这个代码后就马上去执行主窗体关闭的代码)