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The first thing the ASP.NET page does is set the content type to MS Excel. It then opens the XML file as an XML data document, transforms it with XSL, and sends it to the client's browser.

<%@ Page Language="c#" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Xml" %>
<%@ Import Namespace="System.Xml.Xsl" %>
<script language="C#" runat="server">

    public void Page_Load(Object sender, EventArgs E) {
        Response.ContentType = "application/vnd.ms-excel";
        Response.Charset = "";
        DataSet ds = new DataSet();
        ds.ReadXml(Server.MapPath("Authors.xml"));
        XmlDataDocument xdd = new XmlDataDocument(ds);
        XslTransform  xt = new XslTransform();
        xt.Load(Server.MapPath("Excel.xsl"));
        xt.Transform(xdd, null, Response.OutputStream);
        Response.End();              
      }

</script>