Dependency injection

Register the logger in a host

AddXunit adds the provider to a logging builder:

using var host = new HostBuilder()
    .ConfigureLogging(builder =>
    {
        builder.AddXunit();
    })
    .Build();

Use the parameterless overload to write to whichever test is running, and pass an ITestOutputHelper only when you need to target a specific test. Calling AddXunit() several times registers a single provider, while each call that takes an argument registers one more.

ITestOutputHelper and XUnitLoggerOptions are both optional reference types, so AddXunit(null) is ambiguous and does not compile. Call AddXunit(), or name the argument (AddXunit(testOutputHelper: null)) when you pass a variable that may be null. The same applies to the constructors of XUnitLoggerProvider, which you can also register yourself as a singleton ILoggerProvider.

Filter the entries

The provider alias is XUnit. The filters of the logging configuration apply as for any provider, so SetMinimumLevel sets the levels written to the test output, and the configuration can filter this provider alone:

{
  "Logging": {
    "XUnit": {
      "LogLevel": {
        "Default": "Warning"
      }
    }
  }
}

Test an ASP.NET Core application

With a WebApplicationFactory (from the Microsoft.AspNetCore.Mvc.Testing package), add the logger in the logging builder of the application:

public class UnitTest1(ITestOutputHelper testOutputHelper)
{
    [Fact]
    public async Task Test1()
    {
        using var application = new WebApplicationFactory<Program>();
        using var factory = application.WithWebHostBuilder(builder =>
        {
            builder.ConfigureLogging(builder =>
            {
                // You can override the logging configuration if needed
                //builder.SetMinimumLevel(LogLevel.Trace);
                //builder.AddFilter(_ => true);

                // Register the xUnit logger provider
                builder.AddXunit(testOutputHelper, new XUnitLoggerOptions { IncludeScopes = true });
            });
        });

        using var client = factory.CreateClient();
        _ = await client.GetStringAsync(new Uri("/", UriKind.Relative), TestContext.Current.CancellationToken);
    }
}

If the factory outlives the test that created it, for instance when it is shared through a fixture, call builder.AddXunit() without the helper, so each entry goes to the test that is running when it is logged. To also assert on the entries, add the in-memory logger to the same logging builder.