爲什麼在每次回傳時都會保留舊值？

當我點擊另一個下拉列表中的值時，我正在填充下拉列表，例如國家城市的關係，但問題是，它加載相關值upo點擊，但如果我改變父母下拉值，然後它附加舊值，例如如果我選擇美國在父母下拉，然後它加載城市，即紐約，華盛頓，拉斯維加斯等，但如果我點擊巴基斯坦然後它加載巴基斯坦城市加上美國城市，如追加，爲什麼？爲什麼在每次回傳時都會保留舊值？

代碼：

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>

來源

2014-09-02 user3518032