禁用DropDownList的Items

可以先看看效果，被禁用的DropDownList的Items已经无法选取与颜色也变为灰色。

刚开始，DropDownList没有接收从网址传来的参数时，它每一个Item都是可以选取的，一旦接收了值。所对应的DropDownList的item将无法选取而达到禁用。
xxx.aspx:
 <asp:DropDownList ID="DropDownList1" runat="server">
        </asp:DropDownList>
 
xxx.aspx.cs:
  View Code
 protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            Data_Binding();
        }

        if (Request.QueryString["site"] != null)
        {
            foreach (ListItem li in DropDownList1.Items)
            {
                if (li.Text == Request.QueryString["site"].ToString())
                {
                    li.Attributes.Add("disabled", "disabled");
                }
            }
        }

    }

    private void Data_Binding()
    {
        this.DropDownList1.DataSource = Site();
        this.DropDownList1.DataTextField = "key";
        this.DropDownList1.DataValueField = "value";
        this.DropDownList1.DataBind();
    }

    private Dictionary<string, string> Site()
    {
        Dictionary<string, string> site = new Dictionary<string, string>();
        site.Add("Insus.NET cnblogs", "http://insus.cnblogs.com");
        site.Add("Microsoft", "http://www.microsoft.com");
        site.Add("Google", "http://www.google.com");