public supplier_management()
        {
            InitializeComponent();

            if (frm == null) /*بعين قيمة لل form لو قيمتة ب null وهى تكون قيمتها الحالية  */            // .......[4]
            {
                frm = this;
            }


            this.dataGridView1.DataSource = claSup.GET_ALL_Supplier();
        }
        private static supplier_management frm;                                          //  ......................[1]

        static void frm_formclosed(object sender, FormClosedEventArgs e)                 //  ................[2]
        {
            frm = null;
        }
        private void suppliersManagementToolStripMenuItem_Click(object sender, EventArgs e)
        {
            supplier_management frmsupman = new supplier_management();

            frmsupman.ShowDialog();
        }