为什么在每次回传时都会保留旧值？

当我点击另一个下拉列表中的值时，我正在填充下拉列表，例如国家城市的关系，但问题是，它加载相关值upo点击，但如果我改变父母下拉值，然后它附加旧值，例如如果我选择美国在父母下拉，然后它加载城市，即纽约，华盛顿，拉斯维加斯等，但如果我点击巴基斯坦然后它加载巴基斯坦城市加上美国城市，如追加，为什么？为什么在每次回传时都会保留旧值？

代码：

protected void drpAgainstDistrict_SelectedIndexChanged(object sender, EventArgs e)//against district drop down selected index changed event 
    { 
     if (txtSelectedID.Text != "") 
     { 
      MyComplaints obj = new MyComplaints(); 
      int ID = Convert.ToInt32(drpAgainstDistrict.SelectedValue); 
      DataSet ds = obj.SelectHomePoliceStaions(ID); 

      drpAgainstPoliceStation.DataTextField = "PsName"; 
      drpAgainstPoliceStation.DataValueField = "ID"; 
      drpAgainstPoliceStation.DataSource = ds.Tables[0]; 
      drpAgainstPoliceStation.DataBind(); 
      HiddenFieldShowHideButtons.Value = "True"; 
     } 
     else 
     { 
      MyComplaints obj = new MyComplaints(); 
      int ID = Convert.ToInt32(drpAgainstDistrict.SelectedValue); 
      DataSet ds = obj.SelectHomePoliceStaions(ID); 

      drpAgainstPoliceStation.DataTextField = "PsName"; 
      drpAgainstPoliceStation.DataValueField = "ID"; 
      drpAgainstPoliceStation.DataSource = ds.Tables[0]; 
      drpAgainstPoliceStation.DataBind(); 
     } 

    }

的.aspx

<label>Relevant District</label> 
<asp:DropDownList ID="drpAgainstDistrict" runat="server" 
        Width="178px" AutoPostBack="True" 
        OnSelectedIndexChanged="drpAgainstDistrict_SelectedIndexChanged"> 
</asp:DropDownList> 

<label>Relevant Police Station</label> 
<asp:DropDownList ID="drpAgainstPoliceStation" runat="server" 
        Width="178px" AppendDataBoundItems="true"> 
    <asp:ListItem Text="-Select-" Value="-1"></asp:ListItem> 
</asp:DropDownList>

来源

2014-09-02 user3518032