最近在自学 自己做一个小项目玩玩，用到dropdownlist的三级联动,不知道如何实现，请各位指点，谢谢!
代码：
目的是从一张表中选出 "学院" 填充dropdownlist1,然后根据dropdownlist1的选择 从表中选出 "专业"填充dropdownlist2,再根据dropdownlist2的选择，从表中选出"班级" 填充dropdownlist3,但是好像每次回传之后dropdownlist1都是一样的，导致dropdownlist2也是一样..
protected void Page_Load(object sender, EventArgs e)
    {
            string strCon = "Data Source=.;" +
                                  "Initial Catalog=studentManageSystem;" +
                                  "Integrated Security=True;";

            string strSQL = "select distinct academyName from academyCourse";

            SqlConnection con = new SqlConnection(strCon);

            con.Open();

            SqlDataAdapter da = new SqlDataAdapter(strSQL, con);

            DataSet ds = new DataSet();

            da.Fill(ds);

            DropDownList1.DataSource = ds.Tables[0];
            DropDownList1.DataValueField = "academyName";
            DropDownList1.DataTextField = "academyName";
            DropDownList1.DataBind();//触发绑定事件

            con.Close();
        }
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        
            string strCon ="Data Source=.;" +
                                  "Initial Catalog=studentManageSystem;" +
                                  "Integrated Security=True;";

            string strSQL = string.Format("select distinct academyprofession from academyCourse where academyName='{0}'", DropDownList1.SelectedValue.ToString().Trim());

            SqlConnection con = new SqlConnection(strCon);

            con.Open();

            SqlDataAdapter da = new SqlDataAdapter(strSQL, con);

            DataSet ds = new DataSet();

            da.Fill(ds);

            DropDownList2.DataSource = ds.Tables[0];
            DropDownList2.DataValueField = "academyprofession";
            DropDownList2.DataTextField = "academyprofession";
            DropDownList2.DataBind();//触发绑定事件

            con.Close();
        
    }
    protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
    {
        
            string strCon ="Data Source=.;" +
                                  "Initial Catalog=studentManageSystem;" +
                                  "Integrated Security=True;";

            string strSQL = string.Format("select distinct classes from academyCourse where academyprofession='{0}'", DropDownList2.SelectedValue.ToString().Trim());

            SqlConnection con = new SqlConnection(strCon);

            con.Open();

            SqlDataAdapter da = new SqlDataAdapter(strSQL, con);

            DataSet ds = new DataSet();

            da.Fill(ds);

            DropDownList3.DataSource = ds.Tables[0];
            DropDownList3.DataValueField = "classes";
            DropDownList3.DataTextField = "classes";
            DropDownList3.DataBind();//触发绑定事件

            con.Close();

Page_Load中加IsPostBack