小弟现在在做一个省市区联动遇到了一个问题，就是第一次选择省的时候可以出现市，选择市的时候，可以出现区，但是我又重新选择省后出现问题了
绑定得到省的代码：

 AreasData Sarea = new AreasData();
            ExecObj ordExecObj = CN56NET.LPP.Members.GetBusiness.InitClass.GetLocalMembersAreasSystem().GetAreasByParentCode(ref Sarea, PubWinClass.ConnObj, "Directory.Areas");
            if (ordExecObj.ExecFlag)
            {
                Cob_StartProvince.DataSource = Sarea.Tables[0];
                Cob_StartProvince.DisplayMember = "AreaName";
                Cob_StartProvince.ValueMember = "AreaCode";
            }

变动省选择项后联动事件

Cob_StartCity.Items.Clear();
            Cob_StartCounty.Items.Clear();
          
                AreasData Sarea = new AreasData();
                ExecObj ordExecObj = CN56NET.LPP.Members.GetBusiness.InitClass.GetLocalMembersAreasSystem().GetAreasByParentCode(ref Sarea, PubWinClass.ConnObj, Cob_StartProvince.SelectedValue.ToString());
                if (ordExecObj.ExecFlag)
                {
                    Cob_StartCity.DataSource = Sarea.Tables[0];
                    Cob_StartCity.DisplayMember = "AreaName";
                    Cob_StartCity.ValueMember = "AreaCode";
                }
           

变动市选择项后联动事件

Cob_StartCounty.Items.Clear();
          
                AreasData Sarea = new AreasData();
                ExecObj ordExecObj = CN56NET.LPP.Members.GetBusiness.InitClass.GetLocalMembersAreasSystem().GetAreasByParentCode(ref Sarea, PubWinClass.ConnObj, Cob_StartCity.SelectedValue.ToString());
                if (ordExecObj.ExecFlag)
                {
                    Cob_StartCounty.DataSource = Sarea.Tables[0];
                    Cob_StartCounty.DisplayMember = "AreaName";
                    Cob_StartCounty.ValueMember = "AreaCode";
                }
           

第一次成功但是再此选择出现

怎么断开？