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Start with xUnit 3 or 4

Create a directory for your tests and save this as Tests.csproj. This example selects xUnit 4; the substitutions for xUnit 3 are listed below.

Tests.csproj
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <OutputType>Exe</OutputType>
    <IsTestProject>true</IsTestProject>
    <IsPackable>false</IsPackable>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="ProTest.Avalonia.Headless.XUnit.v4" Version="12.1.4" />
    <PackageReference Include="xunit.v3.mtp-off" Version="4.0.2" />
    <PackageReference Include="xunit.runner.visualstudio" Version="4.0.1" PrivateAssets="all" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.11.0" />
  </ItemGroup>
</Project>

For xUnit 3, use ProTest.Avalonia.Headless.XUnit.v3 version 12.1.4 and xunit.v3.mtp-off version 3.2.2. Set the target framework to net8.0 if your tests run on .NET 8. Both versions support the same attributes and application setup.

Configure the application​

TestAppBuilder.cs
using Avalonia;
using Avalonia.Headless;

[assembly: AvaloniaTestApplication(typeof(Tests.TestAppBuilder))]

namespace Tests;

public static class TestAppBuilder
{
    public static AppBuilder BuildAvaloniaApp() =>
        AppBuilder.Configure<Application>()
            .UseHeadless(new AvaloniaHeadlessPlatformOptions());
}

Place the assembly attribute once in the test project. The adapter creates the application and dispatches UI work. Avoid initializing another Avalonia application yourself.

Write facts and theories​

EditorTests.cs
using Avalonia.Controls;
using Avalonia.Headless;
using Avalonia.Headless.XUnit;
using Avalonia.Threading;
using Xunit;

namespace Tests;

public class EditorTests
{
    [AvaloniaFact]
    public async Task Typing_updates_text()
    {
        var editor = new TextBox();
        var window = new Window { Width = 240, Height = 80, Content = editor };
        window.Show();
        try
        {
            editor.Focus();
            window.KeyTextInput("Ready");
            await Task.Yield();
            Assert.True(Dispatcher.UIThread.CheckAccess());
            Assert.Equal("Ready", editor.Text);
        }
        finally
        {
            window.Close();
        }
    }

    [AvaloniaTheory]
    [InlineData("alpha")]
    [InlineData("beta")]
    public void Theory_rows_have_dispatcher_access(string value)
    {
        Assert.True(Dispatcher.UIThread.CheckAccess());
        Assert.Equal(value, new TextBox { Text = value }.Text);
    }
}
dotnet test

Use [AvaloniaFact] and [AvaloniaTheory] for controls, layout, binding, input, or dispatcher operations. Ordinary [Fact] tests remain useful for independent model and service logic.

Add application themes​

Template-dependent controls need the theme used by your application. Add Avalonia.Themes.Fluent at 12.1.4 and configure an application class:

using Avalonia;
using Avalonia.Themes.Fluent;

public sealed class TestApplication : Application
{
    public override void Initialize() => Styles.Add(new FluentTheme());
}

Change the builder to AppBuilder.Configure<TestApplication>(). You can also reuse an application's XAML initialization and styles, provided its startup does not create a separate desktop lifetime.

Next steps​

Topic reference: Avalonia's xUnit guide. The project configuration above is specific to ProTest's xUnit 3 and 4 packages.