internal static void ReleaseViewModel(string fileName)
 {
     lock (ViewModels)
     {
         if (ViewModels.ContainsKey(fileName))
         {
             ViewModel.ViewModelController viewModelController = ViewModels[fileName];
             if (--viewModelController.ReferenceCount == 0)
             {
                 ViewModels.Remove(fileName);
                 viewModelController.Dispose();
             }
         }
     }
 }