Question

当我在下拉列表中选择一个值时，我会收到一个回发，即使在回发后也会选择我选择的值。如何获取默认值＆lt; - 选择城市 - ＆gt;在回发后再次选择值？

HTML：

<asp:DropDownList ID="ddlCity" Width="200px" runat="server" AutoPostBack="true"
 OnSelectedIndexChanged="ddlCity_SelectedIndexChanged"> </asp:DropDownList>

标记：

protected void Page_Load(object sender, EventArgs e)
{
    DataTable CityMembersTable = GetCity();
    ddlCity.DataSource = CityMembersTable;
    ddlCity.DataTextField = "CityName";
    ddlCity.DataValueField = "CityID";
    ddlCity.DataBind();

    ddlCity.Items.Insert(0, new ListItem("<--Choose City-->", ""));
}

protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
{
    if (ddlCity.SelectedValue == "")
    {
        return;
    }

    ddlCity.SelectedValue == "0"; //Dose not work...
}

Answer 1

首先需要在页面加载时处理IsPostBack，因为您在页面加载时绑定下拉列表。此外，如果您的if条件执行，ddlCity.SelectedValue == "0";将无法执行。

 protected void Page_Load(object sender, EventArgs e)
 {
      if(Page.IsPostBack == false)
      {
         DataTable CityMembersTable = GetCity();
         ddlCity.DataSource = CityMembersTable;
         ddlCity.DataTextField = "CityName";
         ddlCity.DataValueField = "CityID";
         ddlCity.DataBind();

         ddlCity.Items.Insert(0, new ListItem("<--Choose City-->", ""));
      }
 }


 protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
 {
    ddlCity.SelectedIndex = 0; //Put this here
    if (ddlCity.SelectedValue == "")
    {
        return;
    }
 }

Answer 2

尝试使用SelectedIndex = 0而不是SelectedValue。在绑定数据后，您已在位置0添加了值。

如果在绑定数据源之前请求是PostBack，最好在Page_Load中添加额外的检查：

if (!Page.IsPostBack)
{
   //bind data
}

Answer 3

如果这是您的确切代码，则将ddl.SelectedValue与“0”进行比较。相反，你想使用assign运算符，= not ==。

如果仍然无效。试试ddlCity.SelectedIndex = 0;

下拉列表回发到默认选定值？

3 个答案: