---

name: managing-wpf-collectionview-mvvm description: "Encapsulates CollectionView in Service Layer to maintain MVVM principles in WPF. Use when implementing filtering, sorting, or grouping while keeping ViewModels free of WPF dependencies."

5.6 MVVM Pattern with CollectionView

Project Structure

The templates folder contains a .NET 9 WPF project example.

Copytemplates/
├── WpfCollectionViewSample.Core/           ← Pure C# models and interfaces
│   ├── Member.cs
│   ├── IMemberCollectionService.cs
│   └── WpfCollectionViewSample.Core.csproj
├── WpfCollectionViewSample.ViewModels/     ← ViewModel (no WPF references)
│   ├── MainViewModel.cs
│   ├── GlobalUsings.cs
│   └── WpfCollectionViewSample.ViewModels.csproj
├── WpfCollectionViewSample.WpfServices/    ← WPF Service Layer
│   ├── MemberCollectionService.cs
│   ├── GlobalUsings.cs
│   └── WpfCollectionViewSample.WpfServices.csproj
└── WpfCollectionViewSample.App/            ← WPF Application
    ├── Views/
    │   ├── MainWindow.xaml
    │   └── MainWindow.xaml.cs
    ├── App.xaml
    ├── App.xaml.cs
    ├── GlobalUsings.cs
    └── WpfCollectionViewSample.App.csproj

5.6.1 Problem Scenario

When a single source collection needs to be filtered with different conditions across multiple Views while adhering to MVVM principles

5.6.2 Core Principles

• ViewModel must not reference WPF-related assemblies (MVVM violation)
• Encapsulate CollectionViewSource access through Service Layer
• ViewModel uses only IEnumerable or pure BCL types

5.6.3 Architecture Layer Structure

CopyView (XAML)
    ↓ DataBinding
ViewModel Layer (uses IEnumerable, no WPF assembly reference)
    ↓ IEnumerable interface
Service Layer (uses CollectionViewSource directly)
    ↓
Data Layer (ObservableCollection<T>)

5.6.4 Implementation Pattern

1. Service Layer (CollectionViewFactory/Store)

Copy// Services/MemberCollectionService.cs
// This class can reference PresentationFramework
namespace MyApp.Services;

public sealed class MemberCollectionService
{
    private ObservableCollection<Member> Source { get; } = [];

    // Factory Method: Create filtered view
    // Returns IEnumerable so ViewModel doesn't know WPF types
    public IEnumerable CreateView(Predicate<object>? filter = null)
    {
        var viewSource = new CollectionViewSource { Source = Source };
        var view = viewSource.View;

        if (filter is not null)
        {
            view.Filter = filter;
        }

        return view; // ICollectionView inherits IEnumerable
    }

    public void Add(Member item) => Source.Add(item);

    public void Remove(Member? item)
    {
        if (item is not null)
            Source.Remove(item);
    }

    public void Clear() => Source.Clear();
}

2. ViewModel Layer

Copy// ViewModel uses only IEnumerable (pure BCL type)
namespace MyApp.ViewModels;

public abstract class BaseFilteredViewModel
{
    public IEnumerable? Members { get; }

    protected BaseFilteredViewModel(Predicate<object> filter)
    {
        // Receives IEnumerable from Service
        Members = memberService.CreateView(filter);
    }
}

// Each filtered ViewModel
public sealed class WalkerViewModel : BaseFilteredViewModel
{
    public WalkerViewModel()
        : base(item => (item as Member)?.Type == DeviceTypes.Walker)
    {
    }
}

// Or use with direct type casting
public sealed class AppViewModel : ObservableObject
{
    public IEnumerable? Members { get; }

    public AppViewModel()
    {
        Members = memberService.CreateView();
    }

    // Manipulate collection with LINQ when needed
    private void ProcessMembers()
    {
        var memberList = Members?.Cast<Member>().ToList();
        // Processing logic...
    }
}

3. Initialize CollectionView from View (Alternative Approach)

This approach keeps ViewModel completely independent from WPF, but requires initialization logic in View's Code-Behind.

Copy// ViewModel - Uses pure BCL only
namespace MyApp.ViewModels;

public sealed partial class MainViewModel : ObservableObject
{
    [ObservableProperty] private ObservableCollection<Person> _people = [];

    private ICollectionView? _peopleView;

    // Injected from View
    public void InitializeCollectionView(ICollectionView collectionView)
    {
        _peopleView = collectionView;
        _peopleView.Filter = FilterPerson;
    }

    private bool FilterPerson(object item)
    {
        // Filtering logic
        return true;
    }
}

// MainWindow.xaml.cs - View's Code-Behind
namespace MyApp.Views;

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        var viewModel = new MainViewModel();
        DataContext = viewModel;

        // Create CollectionViewSource in View layer
        ICollectionView collectionView =
            CollectionViewSource.GetDefaultView(viewModel.People);

        // Inject into ViewModel
        viewModel.InitializeCollectionView(collectionView);
    }
}

Note: This approach requires ViewModel to know the ICollectionView type, so WindowsBase.dll reference is needed. For complete independence, use the Service Layer approach.

5.6.5 Project Structure (Strict MVVM)

CopyMyApp.Models/              // Pure C# models, BCL only

MyApp.ViewModels/         // Pure C# ViewModel
                          // No WPF assembly references
                          // Uses IEnumerable only

MyApp.Services/           // PresentationFramework reference: YES
                          // WindowsBase reference: YES
                          // Uses CollectionViewSource

MyApp.Views/              // References all WPF assemblies

5.6.6 Assembly Reference Rules

Assemblies that ViewModel project must NOT reference:

• ❌ WindowsBase.dll (contains ICollectionView)
• ❌ PresentationFramework.dll (contains CollectionViewSource)
• ❌ PresentationCore.dll

Assemblies that ViewModel project CAN reference:

• ✅ BCL (Base Class Library) types only
• ✅ System.Collections.IEnumerable
• ✅ System.Collections.ObjectModel.ObservableCollection<T>
• ✅ System.ComponentModel.INotifyPropertyChanged

Assemblies that Service project CAN reference:

• ✅ WindowsBase.dll
• ✅ PresentationFramework.dll
• ✅ All WPF-related assemblies

5.6.7 Key Advantages

1. Single source maintenance: All Views share one ObservableCollection
2. Automatic synchronization: Source changes automatically reflect in all filtered Views
3. MVVM compliance: ViewModel is completely independent from UI framework
4. Reusability: Multiple Views can be created with various filter conditions
5. Testability: ViewModel can be unit tested without WPF

5.6.8 Utilizing CollectionView Features in Service Layer

Service Layer can encapsulate various CollectionView features.

Copy// Services/MemberCollectionService.cs
namespace MyApp.Services;

public sealed class MemberCollectionService
{
    private ObservableCollection<Member> Source { get; } = [];

    public IEnumerable CreateView(Predicate<object>? filter = null)
    {
        var viewSource = new CollectionViewSource { Source = Source };
        var view = viewSource.View;

        if (filter is not null)
        {
            view.Filter = filter;
        }

        return view;
    }

    // Create sorted view
    public IEnumerable CreateSortedView(
        string propertyName,
        ListSortDirection direction = ListSortDirection.Ascending)
    {
        var viewSource = new CollectionViewSource { Source = Source };
        var view = viewSource.View;

        view.SortDescriptions.Add(
            new SortDescription(propertyName, direction)
        );

        return view;
    }

    // Create grouped view
    public IEnumerable CreateGroupedView(string groupPropertyName)
    {
        var viewSource = new CollectionViewSource { Source = Source };
        var view = viewSource.View;

        view.GroupDescriptions.Add(
            new PropertyGroupDescription(groupPropertyName)
        );

        return view;
    }

    public void Add(Member item) => Source.Add(item);
    public void Remove(Member? item) { if (item is not null) Source.Remove(item); }
    public void Clear() => Source.Clear();
}

5.6.9 When Applying DI/IoC

Copy// Interface definition (uses pure BCL types only)
namespace MyApp.Services;

public interface IMemberCollectionService
{
    IEnumerable CreateView(Predicate<object>? filter = null);
    void Add(Member member);
    void Remove(Member? member);
    void Clear();
}

// DI container registration
services.AddSingleton<IMemberCollectionService, MemberCollectionService>();

// ViewModel constructor injection
namespace MyApp.ViewModels;

public sealed partial class AppViewModel(IMemberCollectionService memberService)
    : ObservableObject
{
    public IEnumerable? Members { get; } = memberService.CreateView();
}

5.6.10 Practical Recommendations

1. Project separation: Separate ViewModel and Service into different projects
2. Interface usage: Define Services with interfaces for testability
3. Singleton or DI: Manage Services as Singleton or through DI container
4. Naming conventions:
   • MemberCollectionService (Service suffix)
   • MemberViewFactory (Factory suffix)
   • MemberStore (Store suffix)

5.6.11 Grouping UI in XAML

When using grouped CollectionView, display groups using GroupStyle in ListBox or ItemsControl.

XAML - Grouped ListBox:

Copy<ListBox ItemsSource="{Binding GroupedMembers}">
    <!-- Group header template -->
    <ListBox.GroupStyle>
        <GroupStyle>
            <GroupStyle.HeaderTemplate>
                <DataTemplate>
                    <Border Background="#E0E0E0" Padding="5">
                        <StackPanel Orientation="Horizontal">
                            <TextBlock Text="{Binding Name}"
                                       FontWeight="Bold"
                                       FontSize="14"/>
                            <TextBlock Text=" ("
                                       Foreground="Gray"/>
                            <TextBlock Text="{Binding ItemCount}"
                                       Foreground="Gray"/>
                            <TextBlock Text=" items)"
                                       Foreground="Gray"/>
                        </StackPanel>
                    </Border>
                </DataTemplate>
            </GroupStyle.HeaderTemplate>
            <!-- Optional: Group container style -->
            <GroupStyle.ContainerStyle>
                <Style TargetType="{x:Type GroupItem}">
                    <Setter Property="Margin" Value="0,0,0,10"/>
                </Style>
            </GroupStyle.ContainerStyle>
        </GroupStyle>
    </ListBox.GroupStyle>

    <!-- Item template -->
    <ListBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Name}" Padding="10,5"/>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

XAML - Expander Style Grouping:

Copy<ListBox ItemsSource="{Binding GroupedMembers}">
    <ListBox.GroupStyle>
        <GroupStyle>
            <GroupStyle.ContainerStyle>
                <Style TargetType="{x:Type GroupItem}">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type GroupItem}">
                                <Expander IsExpanded="True">
                                    <Expander.Header>
                                        <StackPanel Orientation="Horizontal">
                                            <TextBlock Text="{Binding Name}"
                                                       FontWeight="Bold"/>
                                            <TextBlock Text="{Binding ItemCount,
                                                       StringFormat=' ({0})'}"
                                                       Foreground="Gray"/>
                                        </StackPanel>
                                    </Expander.Header>
                                    <ItemsPresenter/>
                                </Expander>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </GroupStyle.ContainerStyle>
        </GroupStyle>
    </ListBox.GroupStyle>
</ListBox>

Service Layer - Multiple Sort and Group:

Copy// Create view with sorting and grouping combined
public IEnumerable CreateSortedGroupedView(
    string sortProperty,
    ListSortDirection sortDirection,
    string groupProperty)
{
    var viewSource = new CollectionViewSource { Source = Source };
    var view = viewSource.View;

    // Apply sort first, then group
    view.SortDescriptions.Add(new SortDescription(sortProperty, sortDirection));
    view.GroupDescriptions.Add(new PropertyGroupDescription(groupProperty));

    return view;
}

5.6.12 Microsoft Official Documentation

• CollectionViewSource Class
• Data Binding Overview - Collection Views
• How to: Filter Data in a View
• Service Layer Pattern