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//1.利用前台DropDownList的DataSource屬性與後台的ADO.NET配合來做。
後端:
 protected void Page_Load(object sender, EventArgs e)
    {
        Page.DataBind();
    }
    public SqlDataReader DLfrom1()  //這裡是用「public SqlDataReader」
    {
        string ds = WebConfigurationManager.ConnectionStrings["MailToConnectionString"].ConnectionString;
        SqlConnection conn = new SqlConnection(ds);
        SqlDataReader dr = null;
        string dc = "select * from FromSetting ";
        SqlCommand cmd = new SqlCommand(dc, conn);
        conn.Open();
        dr = cmd.ExecuteReader(CommandBehavior.CloseConnection);//加上CommandBehavior.CloseConnection自動關閉連線，就能安心return
        dr.Read();
        return dr;
    }

前台:

 <asp:DropDownList ID="DropDownList1" runat="server" DataSource="<%# DLfrom1()%>"
    DataTextField="useFrom" DataValueField="usePort" >
    </asp:DropDownList>

//2.在後端直接用ADO.NET處理(方式1)(迴圈去read)。
protected void DLfrom()
    {
        string ds = WebConfigurationManager.ConnectionStrings["MailToConnectionString"].ConnectionString;
        SqlConnection conn = new SqlConnection(ds);
        SqlDataReader dr = null;
        string dc = "select * from FromSetting ";
        SqlCommand cmd = new SqlCommand(dc, conn);
        conn.Open();
        dr = cmd.ExecuteReader();
        while (dr.Read())
       {
          DropDownList_from.Items.Add(new ListItem(dr["useFrom"].ToString(), dr["id"].ToString()));    
       }
        cmd.Cancel();
        dr.Close();
        conn.Close();
        conn.Dispose();
        //http://msdn.microsoft.com/zh-tw/library/cc464128(v=vs.71).aspx
    }

//3.在後端直接用ADO.NET處理(方式2)(用DataSource)。
protected void DLfrom()
    {
        
        string ds = WebConfigurationManager.ConnectionStrings["MailToConnectionString"].ConnectionString;
        SqlConnection conn = new SqlConnection(ds);
        SqlDataReader dr = null;
        string dc = "select * from FromSetting ";
        SqlCommand cmd = new SqlCommand(dc, conn);
        conn.Open();
        dr = cmd.ExecuteReader();
        //dr.Read(); 不須加上這句，不然會失效, 因為已有.DataSource了，不須再跑迴圈去read
        DropDownList_from.DataSource = dr;
        DropDownList_from.DataValueField = "id";
        DropDownList_from.DataTextField = "useFrom";
        DropDownList_from.DataBind();
        cmd.Cancel();
        dr.Close();
        conn.Close();
        conn.Dispose();
        //http://msdn.microsoft.com/zh-tw/library/cc464128(v=vs.71).aspx
    }

//4.在後端直接用ADO.NET(方式3)(迴圈去read)。
protected void DLfrom()
    {
        string ds = WebConfigurationManager.ConnectionStrings["MailToConnectionString"].ConnectionString;
        SqlConnection conn = new SqlConnection(ds);
        SqlDataReader dr = null;
        string dc = "select * from FromSetting ";
        SqlCommand cmd = new SqlCommand(dc, conn);
        conn.Open();
        dr = cmd.ExecuteReader();
        while (dr.Read())
       {
           DropDownList_from.DataValueField =dr["id"].ToString();
           DropDownList_from.DataTextField = dr["useFrom"].ToString();
           DropDownList_from.Items.Add(dr["useFrom"].ToString());   
       }
        cmd.Cancel();
        dr.Close();
        conn.Close();
        conn.Dispose();
        //http://msdn.microsoft.com/zh-tw/library/cc464128(v=vs.71).aspx
    }

PS:有 dr.read()就不須再設定.datasourcedd的屬性, 

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