Combobox droplist item picking with query

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C#

combobox

data-binding

i have 2 combo box, comboBox1 item was taken from databinding(table1"Root" contains ID and Root, table2"Parent" contains ID, Parent, RootID). The item list for comboBox2 are depends on selected value from comboBox1.. the problem is, i can't filter the comboBox2.Displaymember.. please help, here is my codes:

private void comboBox2_DropDown(object sender, EventArgs e)
        {
            cn.Open();
            //comboBox2.DataSource = parentBindingSource;
            SqlCommand cb2 = new SqlCommand("SELECT * FROM PARENT WHERE RootID = " + comboBox1.SelectedValue.ToString(), cn);
            SqlDataAdapter dat = new SqlDataAdapter(cb2);
            DataSet ds = new DataSet();
            dat.Fill(ds, "Parent");
            comboBox2.DataSource = ds;
            cn.Close();
        }

Please help..