Simple UrlHelper extension for rendering app-relative content urls with last-modified timestamp appended as a querystring parameter.

gistfile1.cs

using System;
using System.IO;
using System.Web.Mvc;
using System.Web;

namespace Example.Web.Helpers
{
    public static class UrlHelperExtensions
    {
        /// <summary>
        /// Like Url.Content(...) but also appends a timestamp representing the last write date and time
        /// of the file to the querystring. Useful for cache-busting.
        /// </summary>
        /// <param name="helper"></param>
        /// <param name="contentPath"></param>
        /// <returns></returns>
        public static string ContentWithTimestamp(this UrlHelper helper, string contentPath)
        {
            var contentUrl = helper.Content(contentPath);
            try
            {
                var fileLastModifiedTicks = File.GetLastWriteTime(HttpContext.Current.Server.MapPath(contentPath)).Ticks;
                return contentUrl.Contains("?") ? contentUrl + "&" + fileLastModifiedTicks : contentUrl + "?" + fileLastModifiedTicks;
            } 
            catch (Exception ex)
            {
                return contentUrl;
            }
        }
    }
}