A set of extension methods to help with fingerprinting (cache busting) of static resources in ASP.NET MVC views. In your view, just call @Html.GetCacheableUrl("~/css/default.css") and the timestamp will be appended to the query string.

HtmlHelperExtensionMethods.cs

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

namespace Skybrud.Umbraco.ExtensionMethods {
    
    public static class HtmlHelperExtensionMethods {

        public static string GetCacheableUrl(this HtmlHelper helper, string url) {
            return GetCacheableUrl(url);
        }

        public static string GetCacheableUrl<T>(this HtmlHelper<T> helper, string url) {
            return GetCacheableUrl(url);
        }

        public static string GetCacheableUrl(string url) {
            if (String.IsNullOrWhiteSpace(url)) return "";
            if (url.StartsWith("/") && !url.StartsWith("//")) {
                FileInfo file = new FileInfo(HttpContext.Current.Server.MapPath("~" + url));
                if (file.Exists) {
                    long ticks = file.LastWriteTimeUtc.Ticks;
                    return url + (url.Contains("?") ? "&" : "?") + ticks;
                }
                return url;
            }
            return url;
        }

    }

}

@Html.GetCacheableUrl("~/css/default.css") in combination with if (url.StartsWith("/") && !url.StartsWith("//")) doesn't seem to work ;)