C# 下拉列表回发到默认选定值？
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Dropdownlist postback to default selected value?
提问by user1007103
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?
当我在下拉列表中选择一个值时，我会得到一个回发，即使在回发之后，我选择的值也会被选中。如何在回发后再次获得默认值 <--Choose City--> 作为选定值？
HTML:
HTML：
<asp:DropDownList ID="ddlCity" Width="200px" runat="server" AutoPostBack="true"
OnSelectedIndexChanged="ddlCity_SelectedIndexChanged"> </asp:DropDownList>
Markup:
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...
}
采纳答案by Amar Palsapure
You need to handle IsPostBackon page load first, because you are binding dropdown on page load. Also if your ifcondition executes, ddlCity.SelectedValue == "0";will not execute.
您需要先处理IsPostBack页面加载，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";则不会执行。
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;
}
}
回答by Emil Badh
If that is your exact code you are comparing ddl.SelectedValue against "0". Instead you want to use the assign operator, = not ==.
如果这是您的确切代码，那么您将 ddl.SelectedValue 与“0”进行比较。相反，您想使用赋值运算符，= 而不是 ==。
If it still doesn't work. Try ddlCity.SelectedIndex = 0;
如果还是不行。尝试ddlCity.SelectedIndex = 0;
回答by RBDev
Try to use SelectedIndex = 0instead of SelectedValue. You've added the value at position 0 after binding the data.
尝试使用SelectedIndex = 0而不是 SelectedValue。您在绑定数据后在位置 0 添加了值。
It's also better to add an extra check in the Page_Load if the Request is a PostBack before binding the datasource:
如果在绑定数据源之前请求是回发，最好在 Page_Load 中添加额外的检查：
if (!Page.IsPostBack)
{
//bind data
}
