private void buttonItem6_Click(object sender, EventArgs e) { if (!checkTab("Quản lí xe")) { QuanLiXe usercontrol = new QuanLiXe(); usercontrol.Dock = DockStyle.Fill; TabFormItem tab = tabFormControl1.CreateTab("Quản lí xe", "qlx"); tab.Panel.Controls.Add(usercontrol); tabFormControl1.SelectedTab = tab; } }