MDI form control in windows forms

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C#

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I have made a application on windows forms and made mdi form and child forms. I have a reader and its connection has to be closed on form close. I have managed it for Form CLosed Event. But When its closed from mdi form its not.
I want to code on the event of close when closed from the mdi main form. Ihave used this code in mdi :

private Form lastForm;

        private void showForm(Form frm)
        {
            frm.FormClosed += (sender, ea) =>
            {
                if (object.ReferenceEquals(lastForm, sender)) lastForm = null;
            };
            frm.MdiParent = this;
            frm.Show();
            if (lastForm != null) lastForm.Close();
            lastForm = frm;
        }

        private void AddTrayToolStripMenuItem_Click(object sender, EventArgs e)
        {
            showForm(new Add_Tray());
        }

        private void TrayWiseInvenToolStripMenuItem_Click(object sender, EventArgs e)
        {
            showForm(new InventoryTrayWise());
        }

Where to put my code for connection close so that when the the form is closed by the mdi that event performs.