public void AddDishToCategory() { if (SelectedCategoryId == -1) { MessageBox.Show("No category has been selected"); return; } if (SelectedDishInAllDishes == null) { MessageBox.Show("No dish has been selected"); return; } Dish dish = SelectedDishInAllDishes; if (DishesInCategory.Contains(dish)) { MessageBox.Show($"The dish {dish.Name} is already in category {SelectedCategoryId}"); return; } DataAccess da = new DataAccess(); da.AddDishToCategory(dish.Id, SelectedCategoryId, SelectedMenu.Id); DishesInCategory.Add(dish); SelectedDishInCategory = dish; NotifyOfPropertyChange(() => DishesInCategory); }