HomeController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.Mvc;

namespace SampleApp.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }

        public ActionResult DownloadExcel()
        {
            var tableData = new StringBuilder();
            var products = GetProducts();
            foreach(var product in products)
            {
                tableData.Append("<Row ss:AutoFitHeight='0'>");
                tableData.Append("<Cell><Data ss:Type='String'>" + product.ProductName + "</Data></Cell>");
                tableData.Append("<Cell><Data ss:Type='Number'>" + product.UnitPrice + "</Data></Cell>");
                tableData.Append("</Row>");
            }

            // read the excel file template from the resource
            var xmlTemplate=System.IO.File.ReadAllText(Server.MapPath("~/App_Data/ExcelTemplate.xml"));
            //Replace placeholder content
            xmlTemplate = xmlTemplate.Replace("$ROWSPLACEHOLDER$", tableData.ToString());
            var data=System.Text.Encoding.UTF8.GetBytes(xmlTemplate);
            return File(data,"application/vnd.ms-excel", "Products.xml");
        }

        private IEnumerable<Product> GetProducts()
        {
            using (var ctx = new NorthwindEntities())
            {
                return ctx.Products.ToList();
            }
        }
    }
}