DropDownList绑定数据源后,如何设置选中项

DropDownList在绑定数据源控件后,该如何设置选中项呢
我按以下方法未能实现,我的代码如下:

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1"
                DataTextField="subject" DataValueField="id" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
            </asp:DropDownList>

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:strconn %>"
            SelectCommand="SELECT [id], [subject] FROM [web_about]"></asp:SqlDataSource>

CS文件中来设置选中项:

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
         string id=Request.QueryString["id"];
            DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(id));
        }
    }

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        Response.Redirect(Request.Path + "?id=" + DropDownList1.SelectedItem.Value);
    }

但以上的方法在不绑定数据源的情况下正常:
<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"
                    AutoPostBack="True">
                    <asp:ListItem Value="1">选项一</asp:ListItem>
                    <asp:ListItem Value="2">选项二</asp:ListItem>
                </asp:DropDownList>
由于初学.NET,不知是什么原因?