Get Primary Key of the table From Combo Box

Question

I'm using this method to fetch foreign key from Table Teachers. Combo box have teachers name. Whenever I changed the selectoin It shows the name of the selected teacher in txtname.Text. But the message box inside while loop is not popping Up.

private void cmbTeacher_SelectedIndexChanged(object sender, EventArgs e)
        {
            string Tname = cmbTeacher.SelectedItem.ToString(); 
            con.Open();
            string qbc = "Select TeacherID from Teachers where Name = ' " + Tname + " ' ";
            SqlCommand cmda = new SqlCommand(qbc, con);
            SqlDataReader drat = cmda.ExecuteReader();
            while (drat.Read())
            {
                textTeacherID.Text = drat[0].ToString();
                MessageBox.Show(drat[0].ToString());
            }
            con.Close();
            txtname.Text = cmbTeacher.SelectedItem.ToString();
        }

Actually drop down list of Combo box is a column of name from table Teachers. When a user select the teacher from the drop down list I want to fetch the primary key of that teacher. Look at this Code

 // Teacher Table
            con.Open();
            String qt = "Select Name, TeacherID from Teachers";
            SqlCommand cmdt = new SqlCommand(qt, con);
            SqlDataReader drt = cmdt.ExecuteReader();
            while (drt.Read())
            {
                cmbTeacher.Items.Add(drt["Name"].ToString());
                cmbTeacher.DisplayMember = (drt["Name"].ToString());
                cmbTeacher.ValueMember = (drt["TeacherID"].ToString());
                cmbTeacher.Text = "Select Teacher";

            }
            con.Close();

How Can I get the primary key of that Teacher???