private void button_QL_BC_TH_TH_CL_DV_Click(object sender, EventArgs e) { if (formQLBCTHTHCLDV == null || formQLBCTHTHCLDV.IsDisposed) { formQLBCTHTHCLDV = new QuanLyBaoCaoTHTHCLDV(); formQLBCTHTHCLDV.MdiParent = this; formQLBCTHTHCLDV.Show(); } else { formQLBCTHTHCLDV.Activate(); } }