禁用DropDownList的Items

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            Data_Binding();
        }

        if (Request.QueryString["site"] !&＃61; null)
        {
            foreach (ListItem li in DropDownList1.Items)
            {
                if (li.Text &＃61;&＃61; Request.QueryString["site"].ToString())
                {
                    li.Attributes.Add("disabled", "disabled");
                }
            }
        }

    }

    private void Data_Binding()
    {
        this.DropDownList1.DataSource &＃61; Site();
        this.DropDownList1.DataTextField &＃61; "key";
        this.DropDownList1.DataValueField &＃61; "value";
        this.DropDownList1.DataBind();
    }

    private Dictionary<string, string> Site()
    {
        Dictionary<string, string> site &＃61; 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");
        return site;
    }