DROPDOWNLIST回发到默认选中的价值？ [英] Dropdownlist postback to default selected value?

问题描述

当我选择在DropDownList的值，我收到了回发，我选择的值甚至是回发后选定的。我如何获得默认值，＆LT; - 选择城市 - >作为选择的值回传后再次

？

HTML

 ＆LT; ASP：DropDownList的ID =ddlCityWIDTH =200像素=服务器的AutoPostBack =真
 OnSelectedIndexChanged =ddlCity_SelectedIndexChanged＆GT; ＆LT; / ASP：DropDownList的＆GT;
 

标记：

 保护无效的Page_Load（对象发件人，EventArgs的发送）
{
    数据表CityMembersTable = GetCity（）;
    ddlCity.DataSource = CityMembersTable;
    ddlCity.DataTextField =CITYNAME;
    ddlCity.DataValueField =CityID;
    ddlCity.DataBind（）;    ddlCity.Items.Insert（0，新的ListItem（＆LT;  - 选择城市 - ＆gt;中，））;
}保护无效ddlCity_SelectedIndexChanged（对象发件人，EventArgs的发送）
{
    如果（ddlCity.SelectedValue ==）
    {
        返回;
    }    ddlCity.SelectedValue ==0; //没有作用？
}
 

解决方案

您需要处理的IsPostBack 在页面加载第一，因为你是在页面加载绑定下拉列表。此外，如果你的如果执行情况， ddlCity.SelectedValue ==0; 不会执行

 保护无效的Page_Load（对象发件人，EventArgs的发送）
 {
      如果（Page.IsPostBack == FALSE）
      {
         数据表CityMembersTable = GetCity（）;
         ddlCity.DataSource = CityMembersTable;
         ddlCity.DataTextField =CITYNAME;
         ddlCity.DataValueField =CityID;
         ddlCity.DataBind（）;         ddlCity.Items.Insert（0，新的ListItem（＆LT;  - 选择城市 - ＆gt;中，））;
      }
 }
 保护无效ddlCity_SelectedIndexChanged（对象发件人，EventArgs的发送）
 {
    ddlCity.SelectedIndex = 0; //将这个位置
    如果（ddlCity.SelectedValue ==）
    {
        返回;
    }
 }
 

When I select a value in the dropdownlist I get a postback and the value I selected is selected even after the postback. How do I get the default value, <--Choose City--> as selected value again after the postback?

HTML:

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

Markup:

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...
}

解决方案

You need to handle IsPostBack on page load first, because you are binding dropdown on page load. Also if your if condition executes, ddlCity.SelectedValue == "0"; will not execute.

 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;
    }
 }

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