Avalonia Testing

Avalonia Testing

Overview

Two testing approaches: ViewModel unit tests (no Avalonia needed, plain xUnit/NUnit), and headless UI tests using Avalonia.Headless (renders without a display).

ViewModel Unit Tests (No UI)

No special setup needed — ViewModels are plain C# classes:

[Fact]
public void Name_Change_RaisesPropertyChanged()
{
    var vm = new MainViewModel();
    var raised = new List();
    vm.PropertyChanged += (_, e) => raised.Add(e.PropertyName);

    vm.Name = "Alice";

    Assert.Contains(nameof(vm.Name), raised);
    Assert.Equal("Alice", vm.Name);
}

[Fact]
public async Task SaveCommand_WhenNameEmpty_CannotExecute()
{
    var vm = new MainViewModel { Name = "" };
    Assert.False(vm.SaveCommand.CanExecute(null));
}

Headless Testing Setup

NuGet packages:

• Avalonia.Headless
• Avalonia.Headless.XUnit (or .NUnit)

// Assembly-level attribute (AssemblyInfo.cs or top of test file)
[assembly: AvaloniaTestApplication(typeof(TestAppBuilder))]

public class TestAppBuilder
{
    public static AppBuilder BuildAvaloniaApp()
        => AppBuilder.Configure()
            .UseHeadless(new AvaloniaHeadlessOptions { UseHeadlessDrawing = true });
}

Writing Headless Tests (xUnit)

public class MainWindowTests
{
    [AvaloniaFact]
    public void Button_Click_UpdatesViewModel()
    {
        var window = new MainWindow();
        var vm = new MainViewModel();
        window.DataContext = vm;
        window.Show();

        var button = window.FindControl("SaveButton")!;
        button.RaiseEvent(new RoutedEventArgs(Button.ClickEvent));

        Assert.True(vm.SaveWasCalled);
    }

    [AvaloniaFact]
    public async Task TextBox_Input_UpdatesBinding()
    {
        var window = new MainWindow();
        window.DataContext = new MainViewModel();
        window.Show();

        var textBox = window.FindControl("NameBox")!;
        textBox.RaiseTextInput("Hello");

        await Task.Delay(50); // allow binding to propagate
        Assert.Equal("Hello", ((MainViewModel)window.DataContext!).Name);
    }
}

Finding Controls

// By name (x:Name)
var btn = window.FindControl("MyButton");

// By type (first match)
var textBox = window.FindDescendantOfType();

// All of type
var allButtons = window.GetVisualDescendants().OfType();

Simulating Input

// Keyboard
window.KeyPress(Key.Enter, KeyModifiers.None);
window.KeyTextInput("Hello");

// Mouse/Pointer
window.MouseDown(new Point(100, 50), MouseButton.Left);
window.MouseMove(new Point(150, 50));
window.MouseUp(new Point(150, 50), MouseButton.Left);

Screenshot Testing

[AvaloniaFact]
public void Window_RendersCorrectly()
{
    var window = new MainWindow();
    window.Show();

    // Capture frame as bitmap
    var bitmap = window.CaptureRenderedFrame()!;
    // Compare or save for regression
    bitmap.Save("snapshot.png");
}

Testing with NUnit

[TestFixture]
public class Tests
{
    [AvaloniaTest]  // instead of [Test]
    public void MyTest() { }
}

Community Testing Libraries

| Library | NuGet | Purpose | |---|---|---| | Verify.Avalonia | Verify.Avalonia | Extends Verify for snapshot/approval testing of Avalonia UI |

Snapshot Testing with Verify.Avalonia

[AvaloniaFact]
public async Task Window_MatchesSnapshot()
{
    var window = new MainWindow();
    window.Show();

    await Verify(window); // saves .verified.png on first run, diffs on subsequent
}

Requires: Verify.Avalonia + Verify.Xunit (or NUnit variant). On first run, creates a .verified.png baseline. Subsequent runs diff against it — fails on visual regression.

Common Mistakes

• Missing [assembly: AvaloniaTestApplication] — headless renderer not initialized, NullReferenceException
• Using [Fact] instead of [AvaloniaFact] for UI tests — no Avalonia dispatcher, random failures
• Forgetting window.Show() before interacting — controls not attached to visual tree
• Testing bindings synchronously — bindings may be async, add small delay or use Dispatcher.UIThread.RunJobs()

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: linuxdevel
• Source: linuxdevel/Avalonia-skills
• License: MIT

Install and usage instructions live in the source repository linked above.