Creating UserControl Via Webpart. i need css file import but how?

Question

Everything is ok. i van see StyleSheet.css in page source.But css effect can not run my gridview. How can i do that

 public class MyGridView : WebPart
    {
        GridView gvCustomers;

        protected override void CreateChildControls()
        {
            gvCustomers = new GridView();
            HtmlHead head = (HtmlHead)Page.Header;
            HtmlLink link = new HtmlLink();
            link.Attributes.Add("href", Page.ResolveClientUrl("StyleSheet.css"));
            link.Attributes.Add("type", "text/css");
            link.Attributes.Add("rel", "stylesheet");
            head.Controls.Add(link);

            gvCustomers.CssClass = "tablestyle";
            gvCustomers.HeaderStyle.CssClass = "headerstyle";
            gvCustomers.AlternatingRowStyle.CssClass = "altrowstyle";
            gvCustomers.RowStyle.CssClass = "rowstyle";
            this.Controls.Add(gvCustomers);
        }

        public object DataSource
        {
            get {
                this.EnsureChildControls();
                return gvCustomers.DataSource; }
            set {
                this.EnsureChildControls();

                gvCustomers.DataSource = value; }
        }

Answer 1

Have you tried a ScriptManagerProxy control? using this control within your user control can determine if the CSS has already been added to your page, and only add one reference to your css file