如何添加默认的"选取[选择这个ASP.NET DropDownList控件？ [英] How to add a default "Select" option to this ASP.NET DropDownList control?

问题描述

我是一个新的ASP.NET开发人员，我想了解LINQ到实体。我试图与LINQ的声明一个DropDownList绑定检索在状态实体状态列表。一切工作正常。不过，我现在尝试添加，选择选项DropDownList的，但它不会和我一起工作。
你能告诉我如何解决这个

的 ASP.NET代码：的

 < ASP：DropDownList的ID =DropDownList1=服务器的AutoPostBack =真
 OnSelectedIndexChanged =DropDownList1_SelectedIndexChanged > 
  

的代码隐藏：的

 保护无效的Page_Load（对象发件人，EventArgs五）
 {
如果（！的IsPostBack）
 {
 DropDownList1.Items 。新增（新的ListItem（选择，0，真））; 
 bindStatusDropDownList（）; 
} 
} 
 
私人无效bindStatusDropDownList（）
 {
状态状态=新状态（）; 
 DropDownList1.DataSource = status.getData（）; 
 DropDownList1.DataValueField =ID; 
 DropDownList1.DataTextField =说明; 
 DropDownList1.DataBind（）; 
} 
  

的 更新：的

我也试过在标记设置DropDownList的做，但它并没有跟我

<$ p工作过$ p> < ASP：DropDownList的ID =DropDownList1=服务器的AutoPostBack =真
OnSelectedIndexChanged =DropDownList1_SelectedIndexChanged>
< ASP：列表项选择=真值=0文本=选择>< / ASP：ListItem的>
< / ASP：DropDownList的>


解决方案

它不工作的原因是因为您要添加一个项目列表，然后重写整个列表用新的数据源这将清除并重新填充您的列表，失去了先手动添加的项目。

所以，你需要做的反向是这样的：

 状态状态=新状态（）; 
 DropDownList1.DataSource = status.getData（）; 
 DropDownList1.DataValueField =ID; 
 DropDownList1.DataTextField =说明; 
 DropDownList1.DataBind（）; 
 
 //添加您的第一个项目
 DropDownList1.Items.Insert（0，选择）; 
  

I am a new ASP.NET developer and I am trying to learn Linq-To-Entities. I am trying to bind a DropDownList with the Linq statement for retrieving the list of status in the Status Entity. Everything is working fine. However, I am trying now to add "Select" option to the DropDownList but It doesn't work with me. Could you please tell me how to fix this?

ASP.NET Code:

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"
                OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">

Code-Behind:

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DropDownList1.Items.Add(new ListItem("Select", "0", true));
            bindStatusDropDownList();
        }
    }

private void bindStatusDropDownList()
    {
        Status status = new Status();
        DropDownList1.DataSource = status.getData();
        DropDownList1.DataValueField = "ID";
        DropDownList1.DataTextField = "Description";
        DropDownList1.DataBind();
    }

UPDATE:

I also tried to do in the markup set of the DropDownList but it did not work too with me

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"
                OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                    <asp:ListItem Selected="True" Value="0" Text="Select"></asp:ListItem>
            </asp:DropDownList>

解决方案

The reason it is not working is because you are adding an item to the list and then overriding the whole list with a new DataSource which will clear and re-populate your list, losing the first manually added item.

So, you need to do this in reverse like this:

Status status = new Status();
DropDownList1.DataSource = status.getData();
DropDownList1.DataValueField = "ID";
DropDownList1.DataTextField = "Description";
DropDownList1.DataBind();

// Then add your first item
DropDownList1.Items.Insert(0, "Select");

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