At the first glance - replace this:
cbClient.Items.Add(DS.Tables[0].Rows[i][0] + "|" + DS.Tables[0].Rows[i][1]);
with this:
cbClient.Items.Add(ds.Tables[0].Rows[i][0] + "|" + DS.Tables[0].Rows[i][1]);
due to the fact that C# is case-sensitive and you've declared:
DataSet ds = new DataSet();