protected override void OnNavigatedTo(NavigationEventArgs e) { parameters = (Dictionary <string, ViewModelBase>)e.Parameter; TopicViewModel topicVm = (TopicViewModel)parameters["topicVm"]; TopicTitle = topicVm.TopicTitle; //TopicViewModel topicVm = (TopicViewModel) e.Parameter; MessageViewModel messageVM = new MessageViewModel(); DataContext = messageVM; SystemNavigationManager.GetForCurrentView().BackRequested += MainPage_BackRequested; //ObservableCollection<ViewModelBase> ret = await ((MessageViewModel)DataContext).GetListMessage(topicVm.TopicId); messageVM.GetListMessage(topicVm.TopicId); }