注意：

    .如果用事件的话就要把控件的AutoPostBack设置成true

    .防止网页刷新用一个判断    if (!IsPostBack)//判断是第一个开始还是取的返回值

                                {

                                ｝

下拉列表：DropDownList

    .绑定数据：

          //指定数据源

             DropDownList1.DataSource = context.Nation;

             DropDownList1.DataValueField = "Code";

             DropDownList1.DataTextField = "Name";

        //绑定数据源

             DropDownList1.DataBind();

        //新建一个集合，往dropdownlist里面添加集合

            ListItem item = new ListItem();

            item.Text = "中国";

            item.Value = "";

            DropDownList1.Items.Add(item);

        也可以在dropdownlist上面选择数据源

    .取选中项的值

        DropDownList1.SelectedValue.ToString();

    .设置哪一项选中

        DropDownList1.SelectedIndex = ;

        //便利

            foreach (ListItem item in DropDownList1.Items)

            {

                if (item.Value == "n002")

                {

                    item.Selected = true;

                }

            }

 三级联动

  private DataClassesDataContext context = new DataClassesDataContext();

    protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            FillSheng();

            FillShi();

            FillQu();

        }

    }

    public void FillSheng()

    {

        DropDownList1.DataSource = context.ChinaStates.Where(r => r.ParentAreaCode == "");

        DropDownList1.DataValueField = "AreaCode";

        DropDownList1.DataTextField = "AreaName";

        DropDownList1.DataBind();

    }

    public void FillShi()

    {

        string sheng = DropDownList1.SelectedValue.ToString();

        DropDownList2.DataSource = context.ChinaStates.Where(r => r.ParentAreaCode == sheng);

        DropDownList2.DataValueField = "AreaCode";

        DropDownList2.DataTextField = "AreaName";

        DropDownList2.DataBind();

    }

    public void FillQu()

    {

        string shi = DropDownList2.SelectedValue.ToString();

        DropDownList3.DataSource = context.ChinaStates.Where(r => r.ParentAreaCode == shi);

        DropDownList3.DataValueField = "AreaCode";

        DropDownList3.DataTextField = "AreaName";

        DropDownList3.DataBind();

    }

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)

    {

        FillShi();

    }

    protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)

    {

        FillQu();

    }