private void btn_ChoThue_Click(object sender, EventArgs e) { SetbtnColor(sender as Button); if (!CheckExistForm("Form_ChoThue")) { ChoThue.MdiParent = this; ChoThue.StartPosition = FormStartPosition.Manual; ChoThue.Width = 1156; ChoThue.Height = 802; ChoThue.Show(); } else { ActiveChildForm("Form_ChoThue"); } }