填充下拉列表控件并显示“选择一个”作为默认

Question

我有下面的代码块，如何显示“选择一个”作为顶部选定的值？

<table> 
    <tr> 
     <td> 
      <asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="SqlDataSource2" DataTextField="StudentName" DataValueField="StudentName"></asp:DropDownList> 
      <asp:SqlDataSource ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:ddl_connStudentProfile %>" SelectCommand="SELECT [StudentName] FROM [StudentProfile]"></asp:SqlDataSource> 
     </td> 
    </tr> 
</table> 

Answer 1

您可以使用以下内容使所需的选择。你可以在page_Load事件中写这个。

protected void Page_Load(object sender, EventArgs e) 
{ 
    if (!Page.IsPostBack) 
    DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByText("select one")); 
} 

Answer 2

使用列表项

<asp:ListItem Text="Select One" Value="-1"></asp:ListItem> 

，并使用该属性AppendDataBoundItems="true"

结合两种

<asp:DropDownList ID="DropDownList1" runat="server" 
    DataSourceID="SqlDataSource2" DataTextField="StudentName" 
    DataValueField="StudentName" AppendDataBoundItems="true"> 
    <asp:ListItem Text="Select One" Value="-1"></asp:ListItem> 
</asp:DropDownList> 
<asp:SqlDataSource ID="SqlDataSource2" runat="server" 
     ConnectionString="<%$ ConnectionStrings:ddl_connStudentProfile %>" 
     SelectCommand="SELECT [StudentName] FROM [StudentProfile]"> 
</asp:SqlDataSource> 

Answer 3

您可以手动添加此默认项，并设置AppendDataBoundItems到true：

<asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="SqlDataSource2" 
     AppendDataBoundItems="True" 
     DataTextField="StudentName" 
     DataValueField="StudentName"> 
    <asp:ListItem Selected="True" Value="0">select one</asp:ListItem> 
</asp:DropDownList>