FileSystemBase.cs

    public abstract class FileSystemBase
    {
        protected Dictionary<string, File> _cachedFiles;

        protected FileSystemBase()
        {
            _cachedFiles = new Dictionary<string, File>();
        }

        public abstract File CreateFile(string filename);


        public abstract void DeleteFile(string filename);


        public abstract File GetFile(string filename);
        

        public File GetOrCreateFile(string filename)
        {
            var file = GetFile(filename);

            if (file == null)
                file = CreateFile(filename);

            return file;
        }

        public File GetFileCached(string filename)
        {
            if (!_cachedFiles.ContainsKey(filename))
            {
                _cachedFiles.Add(filename,GetFile(filename));
            }

            return _cachedFiles[filename];
        }
    }