這流程沒錯，你把
protected void drp_dept_SelectedIndexChanged(object sender, EventArgs e)
{
string str_dr2 = "select * from lab_major where dept_ID='"+drp_dept.SelectedValue.ToString()+"'";
co_class_add.dr_Bind(str_dr2,drp_major);
drp_major.DataBind();
}
裡面的co_class_add.dr_Bind(str_dr2,drp_major);這個方法拿出來我看看。

Page_Load裡面的代碼也貼出來看看。

你有選擇的時候，是兩個下拉框都沒有變呢？還是改變選擇的那個變了，而連動的那個沒變？

public void dr_Bind(string strCmd, DropDownList dr)
{
SqlDataAdapter adp = new SqlDataAdapter(strCmd, myConnection);
DataSet ds = new DataSet();
adp.Fill(ds);
dr.DataSource = ds;
return;
}

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
string str_dept = "select * from lab_dept";
string str_major = "select * from lab_major";
string str_grade = "select * from lab_grade";
co_class_add.dr_Bind(str_dept, drp_dept);
drp_dept.DataBind();
co_class_add.dr_Bind(str_major, drp_major);
drp_major.DataBind();
co_class_add.dr_Bind(str_grade, drp_grade);
drp_grade.DataBind();
if (Request["Action"] == "edit")
{
string num = Request["Class_num"];
string sqlstr = "select * from lab_class where class_num like " + num + "";
SqlConnection sqlconn = new SqlConnection(ConfigurationManager.ConnectionStrings["labConnectionString"].ConnectionString);
SqlDataAdapter da = new SqlDataAdapter(sqlstr, sqlconn);
DataSet ds = new DataSet();
da.Fill(ds);
txt_class.Text = ds.Tables[0].Rows[0]["class_ID"].ToString();
txt_num.Text = ds.Tables[0].Rows[0]["class_stunum"].ToString();
txt_bizhu.Text = ds.Tables[0].Rows[0]["class_bizhu"].ToString();
drp_dept.SelectedValue = ds.Tables[0].Rows[0]["dept_ID"].ToString();
drp_major.SelectedValue = ds.Tables[0].Rows[0]["major_ID"].ToString();
drp_grade.SelectedValue = ds.Tables[0].Rows[0]["class_grade"].ToString();
}
}
}

public void dr_Bind(string strCmd, DropDownList dr)
{
SqlDataAdapter adp = new SqlDataAdapter(strCmd, myConnection);
DataSet ds = new DataSet();
adp.Fill(ds);
dr.DataSource = ds;

/*******
你這還有兩個屬性呢？
/*********
return;
}

放到程序里绑定了

drpTown.DataSource=DS1.Tables["城鎮名稱"];
drpTown.DataTextField="TownName";//顯示的字段
drpTown.DataValueField="TownCode";//值的字段
drpTown.DataBind();

我沒有看到你那兩個屬性賦值，肯定是這裡你漏了，然後你在selectvalue根本得不到值。

可我在程序里写了啊你看
<asp:DropDownList ID="drp_dept" runat="server" Width="115px" DataTextField="dept_name" DataValueField="dept_ID" OnSelectedIndexChanged="drp_dept_SelectedIndexChanged">
</asp:DropDownList>
<asp:DropDownList ID="drp_major" runat="server" Width="115px" DataTextField="major_name" DataValueField="major_ID">
</asp:DropDownList>

咦，這裡我還沒看到。你這樣，你跟蹤到那個SQL語句，看那個drp_dept的SelectedValue的值是不是正確的。