dropdownlist的动态绑定数据然后怎么添加默认项“请选择”以及如何清空成“请选择”

有两种方法：一种是用dual表查空值，然后绑定。另一种是先动态绑定数据后，然后插入一列默认项。

eg1:

string selectQuery = "select  null extattrid,null extattrname from dual union all select  extattrid,extattrname from VExtAttrDetail where extattrsn in (22)";

     DataSet ds=dboperReq.GetDSet(selectQuery);

    ddlTrantype.DataTextField = "extattrname";

    ddlTrantype.DataValueField="extattrid";

    ddlTrantype.DataSource=ds.Tables[0].DefaultView;

    ddlTrantype.DataBind();

eg2:

 string selectQuery = "select  extattrid,extattrname from VExtAttrDetail where extattrsn in (22)";
    DataSet ds=dboperReq.GetDSet(selectQuery);
    ddlTrantype.DataTextField = "extattrname";
    ddlTrantype.DataValueField="extattrid";
    ddlTrantype.DataSource=ds.Tables[0].DefaultView;
    ddlTrantype.DataBind();
    ddlTrantype.Items.Insert(0,new ListItem(" "," ")); //或者ListItem it = new ListItem("请选择", "0"); 
//mydroplist.Items.Insert(0,it);     或者 ListItem lt = new ListItem(); 
//lt.Text = "请选择"; 
//lt.Value = "00"; 
//lt.Selected = true; 
//ddp.Items.Add(lt);
红色是关键代码，那么如何选择请选择呢？也是两种方式，我比较倾向于后一种方式：
Default.aspx
 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Web;
 5 using System.Web.UI;
 6 using System.Web.UI.WebControls;
 7 
 8 namespace Wolfy.DropDownListDemo
 9 {
10     public partial class Default : System.Web.UI.Page
11     {
12         protected void Page_Load(object sender, EventArgs e)
13         {
14             DropDownListProvince.DataSource = GetList();
15             DropDownListProvince.DataTextField = "Name";
16             DropDownListProvince.DataValueField = "ID";
17             DropDownListProvince.DataBind();
18             RepeaterList.DataSource = GetList();
19             RepeaterList.DataBind();
20         }
21         protected List<Province> GetList()
22         {
23             List<Province> list = new List<Province>();
24             list.Add(new Province() { ID = 1, Name = "北京", Pid = 0 });
25             list.Add(new Province() { ID = 2, Name = "上海", Pid = 0 });
26             list.Add(new Province() { ID = 3, Name = "河南", Pid = 0 });
27             list.Add(new Province() { ID = 4, Name = "河北", Pid = 0 });
28             list.Add(new Province() { ID = 5, Name = "湖南", Pid = 0 });
29             list.Add(new Province() { ID = 6, Name = "湖北", Pid = 0 });
30             return list;
31         }
32 
33         protected void LinkUpdate_Click(object sender, EventArgs e)
34         {
35             LinkButton link = sender as LinkButton;
36             if (link != null)
37             {
38                 int id = Convert.ToInt32(link.CommandArgument);
39                 Province pro = GetList().Find(a => a.ID == id);
40                 this.LiteralID.Text = pro.ID.ToString();
41                 this.LiteralLevel.Text = pro.Pid.ToString();
42                 //DropDownList的index是从零开始的 而绑定的数据的ID是从1开始的，所以为了对应需减一
43                 //改变默认选中项
44                 this.DropDownListProvince.SelectedIndex = pro.ID-1;
45             }
46         }
47     }
48 }
方式二
 1         protected void LinkUpdate_Click(object sender, EventArgs e)
 2         {
 3             LinkButton link = sender as LinkButton;
 4             if (link != null)
 5             {
 6                 int id = Convert.ToInt32(link.CommandArgument);
 7                 Province pro = GetList().Find(a => a.ID == id);
 8                 this.LiteralID.Text = pro.ID.ToString();
 9                 this.LiteralLevel.Text = pro.Pid.ToString();
10                 //方式二
11                 ListItem item = DropDownListProvince.Items.FindByText(pro.Name);
12                 if (item != null)
13                 {
14                     //防止出现多选的情况，将选中项 清除
15                     DropDownListProvince.ClearSelection();
16                     item.Selected = true;
17                 }
18             }
19         }