private void PurchaseWindows_Shown(object sender, EventArgs e)
        {
            frmPurchasebillingwindow                 = new PurchaseBillingWindow();
            frmPurchasebillingwindow.WindowState     = FormWindowState.Maximized;
            frmPurchasebillingwindow.FormBorderStyle = FormBorderStyle.None;
            frmPurchasebillingwindow.TopLevel        = false;

            currentForm = frmPurchasebillingwindow;
            frmPurchasebillingwindow.Size = pnlWindow.Size;
            pnlWindow.Controls.Clear();
            pnlWindow.Controls.Add(frmPurchasebillingwindow);
            frmPurchasebillingwindow.Show();
        }
        private void btnbill_Click(object sender, EventArgs e)
        {
            ApplicationDesign.SetSubSubButtonDesign(ref sender);

            frmPurchasebillingwindow                 = new PurchaseBillingWindow();
            frmPurchasebillingwindow.WindowState     = FormWindowState.Maximized;
            frmPurchasebillingwindow.FormBorderStyle = FormBorderStyle.None;
            frmPurchasebillingwindow.TopLevel        = false;

            currentForm = frmPurchasebillingwindow;
            frmPurchasebillingwindow.Size = pnlWindow.Size;
            pnlWindow.Controls.Clear();
            pnlWindow.Controls.Add(frmPurchasebillingwindow);
            frmPurchasebillingwindow.Show();
        }