Question

我有两个DropDownLists。一旦选择了DropDownList1值，DropDownList2将根据Dropdownlist1进行填充。

但是在页面刷新之后，dropdownlist1选择的值被更改为最顶层的值（DropDownList由数据库填充）。

的.aspx：

<asp:DropDownList ID="DropDownList1" runat="server" Width="300px" CssClass="makeselect" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" AutoPostBack="True" AppendDataBoundItems="true">
</asp:DropDownList>

的.cs：

protected void Page_Load(object sender, EventArgs e) {      

    DataTable subjects = new DataTable();

    if (Page.IsPostBack == false) {
        using (SqlConnection con = new SqlConnection(constring)) {

            SqlDataAdapter adapter = new SqlDataAdapter("SELECT VehicleMake FROM VehicleDB", con);
            adapter.Fill(subjects);

            con.Open();

            DropDownList1.DataSource = subjects;
            DropDownList1.DataTextField = "VehicleMake";
            DropDownList1.DataValueField = "";
            DropDownList1.DataBind();

            con.Close();
        }
    }
}

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) {
    string makename = DropDownList1.SelectedItem.Value;
    keepname = makename;

    Response.Redirect("default.aspx?QSVehicleMake="+makename);
}

Answer 1

您可以将SESSION值用于此目的，使用此值从下拉列表中查找项目

protected void Page_Load(object sender, EventArgs e)
{

        jpyRATE = 1.4;

        DataTable subjects = new DataTable();

        if (Page.IsPostBack == false)
        {
            using (SqlConnection con = new SqlConnection(constring))
            {

                SqlDataAdapter adapter = new SqlDataAdapter("SELECT VehicleMake FROM VehicleDB", con);
                adapter.Fill(subjects);

                con.Open();


                DropDownList1.DataSource = subjects;
                DropDownList1.DataTextField = "VehicleMake";
                DropDownList1.DataValueField = "VehicleMake";
                DropDownList1.DataBind();


                if(!String.IsNullOrEmpty(Session["vehiclemake"] as string)) 
                {
                 DropDownList1.SelectedIndex =
                 DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(Session["vehiclemake"].ToString()));
                }


                con.Close();

            }

  }


    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {

        string makename = DropDownList1.SelectedItem.Value;
        keepname = makename;
        Session["vehiclemake"] = keepname;
        Response.Redirect("default.aspx?QSVehicleMake="+makename);
     }

Answer 2

How to implement cascading DropDownList ASP.Net control?

不要Response.Redirect() SelectedIndexChanged()中的同一页面最好将DropDownList2放在UpdatePanel中以避免整页回发，这是DropDownList1被清除的原因。

Answer 3

这可能是因为您的下拉列表在回发时再次绑定。尝试以下代码。

 protected void Page_Load(object sender, EventArgs e)
        {
          if (!Page.IsPostBack)
                    {
                               //write your dropdownlist1 binding code here
                    }

        }

如何在回发后保持DropDownList1选中的值？

3 个答案: