DropDownList联动简单实例_[Asp.Net教程] 在编程中选中某项的方法：
DropDownList1.ClearSelection();
DropDownList1.Items.FindByValue(st.Year.ToString()).Selected = true;

如果不清空选中项的话，会出现：不能在 DropDownList 中选择多个项。

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
int year = int.Parse(DropDownList1.SelectedValue);
int month = int.Parse(DropDownList2.SelectedValue);
int day = DateTime.DaysInMonth(year, month);
DropDownList3.Items.Clear();
for (int i = 1; i <= day; i++)
{
ListItem newItem = new ListItem(i.ToString(), i.ToString());
DropDownList3.Items.Add(newItem);
}
}
不过我遇到的不是这个问题，是个Bug，关机再开就能运行，奇怪。而且VS2005IDE键位还变了，删除行变成了Ctrl+Y(VB的习惯)，恢复预置环境就好了。

看来遇到问题还是要冷静：可以采用的处理方法是：祈祷一句，骂一句，睡一觉，关机一次。

下面附一个最基本的实例。主要，要实现联动DropDownList1的AutoPostBack属性要开。

Default.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>


















Default.aspx.cs
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (IsPostBack == false)
{
for (int i = 2001; i < 2031; i++)
{
DropDownList1.Items.Add(new ListItem(c(i)));
}
for (int i = 1; i < 13; i++)
{
DropDownList2.Items.Add(new ListItem(c(i)));
}
for (int i = 1; i < 32; i++)
{
DropDownList3.Items.Add(new ListItem(c(i)));
}
}
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList3.Items.Clear();
int days = DateTime.DaysInMonth(int.Parse(DropDownList1.SelectedValue), int.Parse(DropDownList2.SelectedValue));
for (int i = 1; i <= days; i++)
{
DropDownList3.Items.Add(new ListItem(c(i)));
}
}
string c(int i)
{
string u = i.ToString();
if (u.Length == 1) { u = "0" + u; }
return u;
}
}

来源:http://www.cnblogs.com/lqb