UrlHelper extension to append a timestamp to a CSS / JS URL

TimestampExtensions.cs

using System;
using System.IO;
using System.Linq;
using System.Web;
using System.Web.Caching;
using System.Web.Hosting;
using System.Web.Mvc;

namespace NBS.Core.Web.Mvc
{
    public static class UrlHelperExtensions
    {
        private const string CachePrefix = "NBS:Fingerprint:";

        public static string AppendTimestamp(this UrlHelper url, string rootRelativePath)
        {
            if (url is null) throw new ArgumentNullException(nameof(url));
            if (string.IsNullOrWhiteSpace(rootRelativePath)) return string.Empty;

            string cacheKey = CachePrefix + rootRelativePath;
            if (!(HttpRuntime.Cache[cacheKey] is string result))
            {
                string virtualPath = url.Content(rootRelativePath);
                string minPath = url.RequestContext.HttpContext.IsDebuggingEnabled ? null : GetMinPath(virtualPath);

                result = AppendTimestampCore(minPath, out var absolutePath)
                    ?? AppendTimestampCore(virtualPath, out absolutePath)
                    ?? virtualPath;

                if (absolutePath != null)
                {
                    HttpRuntime.Cache.Insert(cacheKey, result, new CacheDependency(absolutePath));
                }
            }

            return result;
        }

        private static string GetMinPath(string virtualPath)
        {
            int extensionIndex = virtualPath.LastIndexOf('.');
            if (extensionIndex == -1) return null;

            if (4 < extensionIndex
                && virtualPath[extensionIndex - 4] == '.'
                && virtualPath[extensionIndex - 3].IsOneOf('M', 'm')
                && virtualPath[extensionIndex - 2].IsOneOf('I', 'i')
                && virtualPath[extensionIndex - 1].IsOneOf('N', 'n'))
            {
                return null;
            }

            return virtualPath.Insert(extensionIndex, ".min");
        }

        private static bool IsOneOf(this char c, [NotNull] params char[] values) => values.Any(v => v == c);

        private static string AppendTimestampCore(string virtualPath, out string absolutePath)
        {
            if (string.IsNullOrWhiteSpace(virtualPath))
            {
                absolutePath = null;
                return null;
            }

            absolutePath = HostingEnvironment.MapPath(virtualPath);
            if (string.IsNullOrWhiteSpace(absolutePath)) return null;

            string sep = virtualPath.IndexOf('?') == -1 ? "?v=" : "&v=";
            DateTime lastWriteTime = File.GetLastAccessTimeUtc(absolutePath);
            return virtualPath + sep + lastWriteTime.Ticks.ToString("x");
        }
    }
}