Combox的去重方法

下拉框去重
dtNew = this.dtNenYoMst.Copy();
drNew = dtNew.NewRow();
drNew["NENRYOU_SBT_CD"] = "";
drNew["NENRYO_SBT_NM"] = "";
dtNew.Rows.InsertAt(drNew, 0);
this.cmbS_NENSYU.DataSource = dtNew.DefaultView.ToTable(true, "NENRYOU_SBT_CD","NENRYO_SBT_NM").Copy();
this.cmbS_NENSYU.ValueMember = "NENRYOU_SBT_CD";
this.cmbS_NENSYU.DisplayMember = "NENRYO_SBT_NM";
this.cmbS_NENSYU.SelectedIndex = -1;