private void MainWindow_Closing(object sender, System.ComponentModel.CancelEventArgs e) { if (_viewModel.ProjectViewModel.Project != null && _viewModel.ProjectViewModel.Project.Dirty) { var result = CustomMessageBox.ShowSavePrompt(); if (result == System.Windows.MessageBoxResult.Cancel) { e.Cancel = true; } else if (result == System.Windows.MessageBoxResult.Yes) { _viewModel.ProjectViewModel.Project.Save(); } } }