Приложение MVVM. Быстрая "Нестрогая MVVM" реализация

private readonly BigViewModel viewModel;
        public AuthorizeView()
        {
            InitializeComponent();
            viewModel = (BigViewModel)DataContext;
        }

private void Search_TextChanged(object sender, TextChangedEventArgs e)
{
    source.Filter -= ftr;
    if (!string.IsNullOrWhiteSpace(search.Text))
    {
        if (IsFilterNotContains(NameSearch))
            ftr += NameSearch;
    }
    else
        ftr -= NameSearch;
    source.Filter += ftr;
}
 
private void Filter_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    source.Filter -= ftr;
    if (filter.SelectedIndex != 0)
    {
        if (IsFilterNotContains(ManufacturerFilter))
            ftr += ManufacturerFilter;
    }
    else
        ftr -= ManufacturerFilter;
    source.Filter += ftr;
}
 
private bool IsFilterNotContains(FilterEventHandler filter)
{
    return ftr is null || !ftr.GetInvocationList().Contains(filter);
}
 
private FilterEventHandler? ftr;
 
private void NameSearch(object sender, FilterEventArgs e)
{
    if (!((Product)e.Item).Name.Contains(search.Text))
        e.Accepted = false;
}
private void ManufacturerFilter(object sender, FilterEventArgs e)
{
    if (((Product)e.Item).Manufacturer != Manufacturer)
        e.Accepted = false;
}
 
private Manufacturer? Manufacturer => filter.SelectedItem as Manufacturer;

    public virtual ObservableCollection<TEntity> ToObservableCollection()
    {
        if (_observable == null)
        {
            _observable = new ObservableCollection<TEntity>(this);
            _observable.CollectionChanged += ObservableCollectionChanged;
            CollectionChanged += LocalViewCollectionChanged;
        }
 
        return _observable;
    }

    public static class WinHelper
    {
        public static void Navigate<TNewView>(this DependencyObject oldView)
            where TNewView : new()
        {
            Window.GetWindow(oldView).Content = new TNewView();
        }
    }

        public void Init(bool recreate = false)
        {
            if (recreate)
            {
                if (File.Exists(dataBaseName))
                    File.Delete(dataBaseName);
            }
            catalog.Database.EnsureCreated();
 
            // Если все сущности загружаются, то не нужно делать Include()
            catalog.Roles.Load();
            catalog.Categories.Load();
            catalog.Manufacturers.Load();
            catalog.Suppliers.Load();
            catalog.Units.Load();
            catalog.Users.Load();
            catalog.Products.Load();
            catalog.OrderStatuses.Load();
            catalog.PickupPoints.Load();
            catalog.Orders.Load();
            catalog.OrderProducts.Load();
        }

        private string? searchText;
        private object searchManufacturer =  ViewsHelper.AllManufacturers;
        public string? SearchText
        {
            get => searchText;
            set { searchText = value; SearchChanged(value?.Trim(), SearchManufacturer as Manufacturer); }
        }
        public object SearchManufacturer
        {
            get => searchManufacturer;
            set { searchManufacturer = value ?? ViewsHelper.AllManufacturers; SearchChanged(SearchText?.Trim(), value as Manufacturer); }
        }
 
        private FilterEventHandler? filter;
        private void SearchChanged(string? text, Manufacturer? manufacturer)
        {
            source.Filter -= filter;
 
            filter = (string.IsNullOrEmpty(text), manufacturer == null) switch
            {
                (false, false) => (s, e) => NameManufacturerSearch(s, e, text!, manufacturer!),
                (false, true) => (s, e) => NameSearch(s, e, text!),
                (true, false) => (s, e) => ManufacturerSearch(s, e, manufacturer!),
                _ => null,
            };
 
            source.Filter += filter;
        }
 
        private static void NameSearch(object sender, FilterEventArgs e, string name)
        {
            e.Accepted = ((Product)e.Item).Name.Contains(name);
        }
        private static void ManufacturerSearch(object sender, FilterEventArgs e, Manufacturer manufacturer)
        {
            e.Accepted = ((Product)e.Item).Manufacturer == manufacturer;
        }
        private static void NameManufacturerSearch(object sender, FilterEventArgs e, string name, Manufacturer manufacturer)
        {
            Product product = (Product)e.Item;
            e.Accepted = product.Manufacturer == manufacturer &&
                         product.Name.Contains(name);
        }