protected void Page_Load(object sender, EventArgs e)
        {
            if (vip)
            {
                System.Web.UI.HtmlControls.HtmlLink hf2 = new System.Web.UI.HtmlControls.HtmlLink();
                hf2.Attributes["type"] = "text/css";
                hf2.Attributes["rel"] = "Stylesheet";
                hf2.Href = "../css/hf2.css";
                this.Header.Controls.Add(hf2);
            }
            else
            {
                System.Web.UI.HtmlControls.HtmlLink hf = new System.Web.UI.HtmlControls.HtmlLink();
                hf.Attributes["type"] = "text/css";
                hf.Attributes["rel"] = "Stylesheet";
                hf.Href = "../css/hf.css";
                this.Header.Controls.Add(hf);
                System.Web.UI.HtmlControls.HtmlLink tanceng = new System.Web.UI.HtmlControls.HtmlLink();
                tanceng.Attributes["type"] = "text/css";
                tanceng.Attributes["rel"] = "Stylesheet";
                tanceng.Href = "../css/tanceng.css";
                this.Header.Controls.Add(tanceng);
            }
        }

------解决方案--------------------

C# code
        bool vip = true;
        HtmlLink link = new HtmlLink();
        link.Attributes.Add("type", "text/css");
        link.Attributes.Add("rel", "stylesheet");
        if (vip)
            link.Attributes.Add("href", "css/vip.css");
        else
            link.Attributes.Add("href", "css/normal.css");
        Page.Header.Controls.Add(link);