private InMemoryDirectoryInfo CreateDirectoryInfo (string fullName, InMemoryDirectoryInfo parent)
 {
   return new InMemoryDirectoryInfo (fullName, parent, _directoryDateTime, _directoryDateTime, _directoryDateTime);
 }
 public IDirectoryInfo[] GetRootDirectories ()
 {
   var rootDirectoryArray = new InMemoryDirectoryInfo[_rootDirectories.Count];
   _rootDirectories.Values.CopyTo (rootDirectoryArray, 0);
   return rootDirectoryArray;
 }