How to encode a XML String with &

I have a XML string that sometimes can contain an &. This is the code (C#):

            var templateDef = String.Format("<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?><template><path>{0}</path><title>{1}</title><description>{2}</description></template>", templateUrl, templateTitle, templateDescription);
            return File(System.Text.Encoding.UTF8.GetBytes(templateDef), "application/octet-stream", templateTitle + ".hdtl");

I have read about HttpUtility.HtmlEncode, but my problem is that I can't encode the string, since it will be encode again in the last line

            return File(System.Text.Encoding.UTF8.GetBytes(templateDef), "application/octet-stream", templateTitle + ".hdtl");

and I can't change this last line. A String.Replace("&","&") could work but is not a proper solution.

Thanks