WPF

Trigger On

• working on WPF UI, MVVM, binding, commands, or desktop modernization
• migrating WPF from .NET Framework to .NET
• integrating newer Windows capabilities into a WPF app
• implementing data binding, styles, templates, or control customization

Documentation

• WPF Overview
• Data Binding Overview
• MVVM Toolkit Introduction
• Styles and Templates
• Migration Guide

References

• patterns.md - MVVM patterns, binding patterns, command patterns, and reusable architectural approaches
• anti-patterns.md - Common WPF mistakes and how to avoid them

Workflow

1. Confirm Windows-only scope — WPF is Windows-only even when the wider .NET stack is cross-platform
2. Apply MVVM pattern — keep views dumb, logic in ViewModels, use commands
3. Manage data binding explicitly — choose correct binding modes, validate at runtime
4. Use styles and templates deliberately — keep UI composable, avoid page-specific hacks
5. Handle threading correctly — use Dispatcher for UI updates, async/await for long operations
6. Validate both designer and runtime — XAML composition failures often surface only at runtime

Project Structure

MyWpfApp/
├── MyWpfApp/
│   ├── App.xaml                # Application entry
│   ├── MainWindow.xaml         # Main window
│   ├── Views/                  # XAML views/windows
│   ├── ViewModels/             # MVVM ViewModels
│   ├── Models/                 # Domain models
│   ├── Services/               # Business logic
│   ├── Converters/             # Value converters
│   ├── Resources/              # Styles, templates, dictionaries
│   └── Controls/               # Custom controls
└── MyWpfApp.Tests/

MVVM Pattern

ViewModel with MVVM Toolkit

public partial class CustomersViewModel : ObservableObject
{
    private readonly ICustomerService _customerService;

    [ObservableProperty]
    private ObservableCollection<Customer> _customers = [];

    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(SaveCommand))]
    private Customer? _selectedCustomer;

    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(RefreshCommand))]
    private bool _isLoading;

    public CustomersViewModel(ICustomerService customerService)
    {
        _customerService = customerService;
    }

    [RelayCommand(CanExecute = nameof(CanRefresh))]
    private async Task RefreshAsync()
    {
        IsLoading = true;
        try
        {
            var items = await _customerService.GetAllAsync();
            Customers = new ObservableCollection<Customer>(items);
        }
        finally
        {
            IsLoading = false;
        }
    }

    private bool CanRefresh() => !IsLoading;

    [RelayCommand(CanExecute = nameof(CanSave))]
    private async Task SaveAsync()
    {
        if (SelectedCustomer is null) return;
        await _customerService.SaveAsync(SelectedCustomer);
    }

    private bool CanSave() => SelectedCustomer is not null;
}

View Binding

<Window x:Class="MyWpfApp.Views.CustomersView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:MyWpfApp.ViewModels"
        d:DataContext="{d:DesignInstance Type=vm:CustomersViewModel}">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <ToolBar Grid.Row="0">
            <Button Content="Refresh"
                    Command="{Binding RefreshCommand}"/>
            <Button Content="Save"
                    Command="{Binding SaveCommand}"/>
        </ToolBar>

        <DataGrid Grid.Row="1"
                  ItemsSource="{Binding Customers}"
                  SelectedItem="{Binding SelectedCustomer}"
                  AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Name"
                                    Binding="{Binding Name}"/>
                <DataGridTextColumn Header="Email"
                                    Binding="{Binding Email}"/>
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>

Dependency Injection

public partial class App : Application
{
    private readonly IHost _host;

    public App()
    {
        _host = Host.CreateDefaultBuilder()
            .ConfigureServices((context, services) =>
            {
                // Services
                services.AddSingleton<ICustomerService, CustomerService>();
                services.AddSingleton<INavigationService, NavigationService>();

                // ViewModels
                services.AddTransient<CustomersViewModel>();
                services.AddTransient<CustomerDetailViewModel>();

                // Views
                services.AddTransient<MainWindow>();
                services.AddTransient<CustomersView>();
            })
            .Build();
    }

    protected override async void OnStartup(StartupEventArgs e)
    {
        await _host.StartAsync();
        var mainWindow = _host.Services.GetRequiredService<MainWindow>();
        mainWindow.Show();
        base.OnStartup(e);
    }

    protected override async void OnExit(ExitEventArgs e)
    {
        await _host.StopAsync();
        _host.Dispose();
        base.OnExit(e);
    }
}

Data Binding Modes

<!-- OneTime: Read once at initialization -->
<TextBlock Text="{Binding CreatedDate, Mode=OneTime}"/>

<!-- OneWay: Source to target only (default for most properties) -->
<TextBlock Text="{Binding Name, Mode=OneWay}"/>

<!-- TwoWay: Bidirectional synchronization -->
<TextBox Text="{Binding Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

<!-- OneWayToSource: Target to source only -->
<TextBox Text="{Binding SearchFilter, Mode=OneWayToSource}"/>

Value Converters

public class BooleanToVisibilityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is bool boolValue)
        {
            return boolValue ? Visibility.Visible : Visibility.Collapsed;
        }
        return Visibility.Collapsed;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value is Visibility.Visible;
    }
}

// Multi-value converter
public class MultiplyConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (values.Length == 2 && values[0] is double a && values[1] is double b)
        {
            return a * b;
        }
        return 0.0;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Styles and Templates

Resource Dictionary

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <!-- Implicit style for all Buttons -->
    <Style TargetType="Button">
        <Setter Property="Padding" Value="10,5"/>
        <Setter Property="Margin" Value="5"/>
        <Setter Property="Background" Value="#0078D4"/>
        <Setter Property="Foreground" Value="White"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="Button">
                    <Border Background="{TemplateBinding Background}"
                            CornerRadius="4"
                            Padding="{TemplateBinding Padding}">
                        <ContentPresenter HorizontalAlignment="Center"
                                          VerticalAlignment="Center"/>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <!-- Named style -->
    <Style x:Key="DangerButton" TargetType="Button" BasedOn="{StaticResource {x:Type Button}}">
        <Setter Property="Background" Value="#D32F2F"/>
    </Style>
</ResourceDictionary>

Threading and Dispatcher

// Update UI from background thread
await Task.Run(async () =>
{
    var data = await LoadDataAsync();

    // Must use Dispatcher to update UI
    Application.Current.Dispatcher.Invoke(() =>
    {
        Items.Clear();
        foreach (var item in data)
        {
            Items.Add(item);
        }
    });
});

// Better: Use async/await properly
private async Task LoadDataAsync()
{
    IsLoading = true;
    try
    {
        // This runs on background thread
        var data = await _service.GetDataAsync();

        // This automatically marshals to UI thread
        Items = new ObservableCollection<Item>(data);
    }
    finally
    {
        IsLoading = false;
    }
}

Anti-Patterns to Avoid

Anti-Pattern	Why It's Bad	Better Approach
Logic in code-behind	Hard to test, tight coupling	Use MVVM with ViewModels
Synchronous blocking calls	UI freezes	Use async/await
Manual INotifyPropertyChanged	Boilerplate, error-prone	Use MVVM Toolkit attributes
Hardcoded colors/sizes	Inconsistent, hard to theme	Use resource dictionaries
Direct Dispatcher.Invoke everywhere	Complex, error-prone	Prefer async/await marshaling
God ViewModel	Unmaintainable	Split into focused ViewModels
Skipping binding validation	Runtime errors hidden	Use ValidatesOnDataErrors
Event handlers for everything	Memory leaks, coupling	Use commands and bindings

Best Practices

1. Use compiled bindings in .NET 5+:

   • Enable

     x:CompileBindings="True"

     for performance

2. Implement INotifyDataErrorInfo for validation:

   [ObservableProperty]
   [NotifyDataErrorInfo]
   [Required(ErrorMessage = "Name is required")]
   [MinLength(2, ErrorMessage = "Name must be at least 2 characters")]
   private string _name = string.Empty;
   

3. Use weak event patterns for long-lived subscriptions:

   WeakEventManager<Source, EventArgs>.AddHandler(source, "EventName", Handler);
   

4. Virtualize large collections:

   <ListBox VirtualizingPanel.IsVirtualizing="True"
            VirtualizingPanel.VirtualizationMode="Recycling"
            ItemsSource="{Binding LargeCollection}"/>
   

5. Freeze Freezables when possible:

   var brush = new SolidColorBrush(Colors.Blue);
   brush.Freeze(); // Thread-safe, better performance
   

6. Use design-time data:

   <Window d:DataContext="{d:DesignInstance Type=vm:MainViewModel, IsDesignTimeCreatable=True}">
   

Testing

[Fact]
public async Task RefreshCommand_LoadsCustomers()
{
    var mockService = new Mock<ICustomerService>();
    mockService.Setup(s => s.GetAllAsync())
        .ReturnsAsync(new[] { new Customer { Name = "Test" } });

    var viewModel = new CustomersViewModel(mockService.Object);

    await viewModel.RefreshCommand.ExecuteAsync(null);

    Assert.Single(viewModel.Customers);
    Assert.Equal("Test", viewModel.Customers[0].Name);
}

[Fact]
public void SaveCommand_CannotExecute_WhenNoSelection()
{
    var mockService = new Mock<ICustomerService>();
    var viewModel = new CustomersViewModel(mockService.Object);

    viewModel.SelectedCustomer = null;

    Assert.False(viewModel.SaveCommand.CanExecute(null));
}

Deliver

• cleaner WPF views and view-model boundaries
• safer binding and threading behavior
• migration guidance grounded in actual Windows constraints
• MVVM pattern with testable ViewModels

Validate

• binding and command flows are explicit
• code-behind is not carrying hidden business logic
• Windows-only assumptions are acknowledged
• threading and dispatcher usage is correct
• styles and resources are properly organized