无法在c＃.net中找到下拉列表索引

Question

我有一个gridview，其中我已经创建了一个面板，该面板有一个状态名称的下拉列表，在选择时应该更新下面列表框的值，该名称为该州的城市名称。 我的问题是我无法调用dropdownlist的selectedIndexchange函数（在禁用viewstatemode时，函数被调用但selectedindex值为null）

 <asp:GridView ID="gvCountry1" runat="server" AutoGenerateColumns="False" 
    DataKeyNames="ID"  OnRowDataBound="gvCountry1_OnRowDataBound" 
    onselectedindexchanged="gvCountry1_SelectedIndexChanged">
    <Columns>
        <asp:TemplateField>
        <ItemTemplate>
        <img alt = "" style="cursor: pointer" src="images/plus.png" />
        <asp:Panel ID="pnlStates" runat="server" style="display:none">
        <asp:DropDownList ID="ddlStates1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlStates1_OnSelectedIndexChanged" >
        <asp:ListItem Text="Select State" Value=""></asp:ListItem>
        </asp:DropDownList><br />
        <asp:ListBox ID="lbCity" runat="server"></asp:ListBox>
        </asp:Panel>
        </ItemTemplate>
        </asp:TemplateField>

后端代码： -

protected void gvCountry1_OnRowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            string countryid = gvCountry1.DataKeys[e.Row.RowIndex].Value.ToString();
            ddlStates1 = (DropDownList)e.Row.FindControl("ddlStates1");
            lbCity=(ListBox)e.Row.FindControl("lbCity");
            SqlDataAdapter adp = new SqlDataAdapter();
            String conString = ConfigurationManager.ConnectionStrings["connection"].ConnectionString;
            con.ConnectionString = conString;
            String conCmd = "sp_SelectStateList";
            SqlCommand cmd = new SqlCommand();
            cmd.CommandText = conCmd;
            cmd.Parameters.AddWithValue("@countryid", SqlDbType.Int).Value = countryid;
            cmd.Connection = con;
            cmd.CommandType = CommandType.StoredProcedure;
            adp.SelectCommand = cmd;
            DataSet ds = new DataSet();
            adp.Fill(ds);
            con.Open();
            ddlStates1.DataSource = cmd.ExecuteReader();
            ddlStates1.DataTextField = "State";
            ddlStates1.DataValueField = "ID";
            ddlStates1.DataBind();
            con.Close();
            ddlStates1.SelectedIndexChanged += ddlStates1_OnSelectedIndexChanged;
            conCmd = "sp_SelectCityList";
            cmd = new SqlCommand();
            cmd.CommandText = conCmd;
            string stateid = ddlStates1.SelectedValue;
            Session["stateid"] = stateid;
            cmd.Parameters.AddWithValue("@stateid", SqlDbType.Int).Value = stateid;
            cmd.Connection = con;
            cmd.CommandType = CommandType.StoredProcedure;
            adp.SelectCommand = cmd;
            ds = new DataSet();
            adp.Fill(ds);
            con.Open();
            lbCity.DataSource = cmd.ExecuteReader();
            lbCity.DataTextField = "City";
            lbCity.DataValueField = "ID";
            lbCity.DataBind();
            con.Close();
            ddlStates1.ViewStateMode = ViewStateMode.Disabled;

        }
    }

    protected void ddlStates1_OnSelectedIndexChanged(object sender, EventArgs e)
    {
        string stateid = ddlStates1.SelectedValue;
        SqlDataAdapter adp = new SqlDataAdapter();
        String conString = ConfigurationManager.ConnectionStrings["connection"].ConnectionString;
        con.ConnectionString = conString;
        String conCmd = "sp_SelectCityList";
        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = conCmd;
        cmd.Parameters.AddWithValue("@stateid", SqlDbType.Int).Value = stateid;
        cmd.Connection = con;
        cmd.CommandType = CommandType.StoredProcedure;
        adp.SelectCommand = cmd;
        DataSet ds = new DataSet();
        adp.Fill(ds);
        con.Open();
        lbCity.DataSource = cmd.ExecuteReader();
        lbCity.DataTextField = "City";
        lbCity.DataValueField = "ID";
        lbCity.DataBind();
        con.Close();
        ddlStates1.ViewStateMode = ViewStateMode.Enabled;
    }

我是c＃.net的新手。在我错的地方帮助我