C# 将项目添加到 DataBound 下拉列表

Adding Item to DataBound Drop Down List

提问by

Yes, I have read most of the topics here, but I can't find an answer that works.

是的，我已经阅读了这里的大部分主题，但找不到有效的答案。

I have Three drop-down lists. The first is databound to grab distinct experiment names. The user selects, page posts back, and the second drop-down menu displays distinct time points. This is where I need help. I need to add an item to THAT drop-down list whose ID, DataTextField, DataValueField are all TimePt.

我有三个下拉列表。第一个是数据绑定以获取不同的实验名称。用户选择，页面回发，第二个下拉菜单显示不同的时间点。这是我需要帮助的地方。我需要向那个下拉列表中添加一个项目，其 ID、DataTextField、DataValueField 都是 TimePt。

Seems simple, but I can't get it to work.

看起来很简单，但我无法让它工作。

protected void Page_Load(object sender, EventArgs e)
        {
            if (Page.IsPostBack)
            {
                TimePt.DataSource = TimePTDD;
                TimePt.DataValueField = "TimePt";
                TimePt.DataTextField = "TimePt";
                TimePt.DataBind();
                TimePt.Items.Insert(0, new ListItem("--Select---", "0"));
                TimePt.SelectedIndex = 0;
            }
        }

I'm missing sometthing.

我错过了一些东西。

回答by Ahmad Mageed

Set AppendDataBoundItems="true"on your dropdown list and it should work.

设置AppendDataBoundItems="true"在您的下拉列表中，它应该可以工作。

Here's a similar question: How to add Item to SqlDataSource databound list

这是一个类似的问题：How to add Item to SqlDataSource databound list

And another one about potential duplicates using this method and a workaround for it: Dropdownlist AppendDataboundItems (first item to be blank)

另一个关于使用此方法的潜在重复项及其解决方法：Dropdownlist AppendDataboundItems（第一项为空）

回答by Ahmad Mageed

<asp:DropDownList ID="ExpAnalysisName" runat="server"
            DataSourceID="DropDownEXP" DataTextField="ExpAnalysisName" 
            DataValueField="ExpAnalysisName" AppendDataBoundItems="true" AutoPostBack=true>
            <asp:ListItem Selected="True" Value="0">Select an Experiment</asp:ListItem>
        </asp:DropDownList>
        <asp:SqlDataSource ID="DropDownEXP" runat="server" 
            ConnectionString="<%$ ConnectionStrings:SGMD3_DataBase %>" SelectCommand="SELECT DISTINCT ExpAnalysisName FROM VW_Data">
        </asp:SqlDataSource>

<asp:DropDownList ID="TimePt" runat="server" AutoPostBack="True" DataSourceID="TimePTDD" DataTextField="TimePt" 
                DataValueField="TimePt">
        </asp:DropDownList>
            <asp:SqlDataSource ID="TimePTDD" runat="server" 
                ConnectionString="<%$ ConnectionStrings:SGMD3_DataBase %>" SelectCommand="SELECT DISTINCT TimePt
    FROM VW_Data
    WHERE ExpAnalysisName = @ExpAnalysisName">
                <SelectParameters>
                    <asp:FormParameter FormField="ExpAnalysisName" Name="ExpAnalysisName" />
                </SelectParameters>
            </asp:SqlDataSource>

So you can have a reference.

回答by Ryan Versaw

I see that you're specifying the DataSourcein two different ways - The DataSourceIDin your markup as well as manually setting the DataSourcein your codebehind. Try removing the DataSourceIDfrom your markup and see if that helps.

我看到您DataSource以两种不同的方式指定-DataSourceID在您的标记中以及DataSource在您的代码隐藏中手动设置。尝试DataSourceID从标记中删除，看看是否有帮助。

It's been a little while since I've used ASP.NET too much, but I have a feeling your DropDownListis rebinding after the Page_Loadwhich would replace your previous binding.

自从我过多地使用 ASP.NET 已经有一段时间了，但我有一种感觉，您DropDownList正在重新绑定，Page_Load这将取代您之前的绑定。

回答by Ben Griswold

My bet is this is a page lifecycle issue. Per MSDN, each data bound control whose DataSourceID property is set calls its DataBind method.

我敢打赌这是一个页面生命周期问题。根据 MSDN，设置了 DataSourceID 属性的每个数据绑定控件都调用其 DataBind 方法。

I think the values are getting bound to the dropdown twice. First, when you manually bind and append the extra item in Page_Load and then the datasource is being bound inside the Page_PreRender event. Try bringing your Page_Load code into Page_PreRender. Hopefully the order helps.

我认为这些值两次绑定到下拉列表。首先，当您在 Page_Load 中手动绑定和附加额外的项目时，然后数据源被绑定在 Page_PreRender 事件中。尝试将您的 Page_Load 代码引入 Page_PreRender。希望订单有所帮助。

回答by Zoran Perokovic

I suggest using OnDataBoundevent of DropDownListcontrol, and put your code behind there. That you way you can combine DataSourceIDwith code behind.

我建议使用控制OnDataBound事件DropDownList，并将您的代码放在那里。这样你就可以DataSourceID与后面的代码结合起来。

回答by Cua

    protected void Page_Load(object sender, EventArgs e)
    {
        if (Page.IsPostBack)
        {
            TimePt.DataValueField = "TimePt";
            TimePt.DataTextField = "TimePt";
            var times = TimePTDD.ToList();
            times.Insert(0, new {TimePt="0",TimePt="--Select--"});
            TimePt.DataSource = times;
            TimePt.DataBind();
            //TimePt.SelectedIndex = 0;
        }
    }