public void ZipCacherShouldCache()
        {
            var cacheKey = Guid.NewGuid().ToString();
            var zipFile = Path.Combine(Directory.GetCurrentDirectory(),"validation.zip");

            var fa = new ZipCacher(zipFile,cacheKey);

            Assert.IsFalse(fa.IsActual());

            var sw = new Stopwatch();

            sw.Start();
            fa.GetContents();
            sw.Stop();

            var firstRun = sw.ElapsedMilliseconds;

            Assert.IsTrue(fa.IsActual());

            sw.Restart();
            fa.GetContents();
            sw.Stop();

            var secondRun = sw.ElapsedMilliseconds;

            Assert.IsTrue(firstRun > secondRun);

            fa = new ZipCacher(zipFile,cacheKey);

            Assert.IsTrue(fa.IsActual());

            sw.Start();
            fa.GetContents();
            sw.Stop();

            var thirdRun = sw.ElapsedMilliseconds;
            Assert.IsTrue(thirdRun < firstRun);

            fa.Clear();
            Assert.IsFalse(fa.IsActual());

            sw.Restart();
            fa.GetContents();
            sw.Stop();

            var fourthRun = sw.ElapsedMilliseconds;
            Assert.IsTrue(fourthRun > secondRun);

            File.SetLastWriteTime(zipFile, DateTime.Now);
            Assert.IsFalse(fa.IsActual());
        }
        /// <summary>
        /// Unpacks zip-files containing the artifact files, and enumerates all (zipped/nonzipped) files.
        /// </summary>
        /// <remarks>This is an expensive operations and should be run once. As well, it unpacks files on the
        /// file system and is not thread-safe.</remarks>
        public void Prepare()
        {
#if !PORTABLE45
            _artifactFiles = new List<string>();

            var zips = Directory.GetFiles(_contentDirectory, "*.zip");

            // Get the files in each *.zip files present in the content directory
            // The ZipCacher will avoid re-extracting these files.
            foreach (string zipPath in zips)
            {
                ZipCacher zc = new ZipCacher(zipPath, CACHE_KEY);
                _artifactFiles.AddRange(zc.GetContents());
            }

            _isPrepared = true;
#else
            throw Error.NotImplemented("File based Core artifact source is not supported on the portable runtime");
#endif

        }