Testing an application

Register the provider

Add an InMemoryLoggerProvider to the logging builder, then read provider.Logs: it collects the entries of every logger the application creates.

// Use with dependency injection
var services = new ServiceCollection();
using var provider = new InMemoryLoggerProvider();
services.AddLogging(builder => builder.AddProvider(provider));

using var serviceProvider = services.BuildServiceProvider();
var logger = serviceProvider.GetRequiredService<ILoggerFactory>().CreateLogger("MyCategory");
logger.LogInformation("Test message");

// Access logs through the provider
var log = Assert.Single(provider.Logs.Informations);
Assert.Equal("Test message", log.Message);

The provider receives the scopes from the logger factory, so the entries have the scopes created by the application, including the ones the factory adds for the current activity when ActivityTrackingOptions is set.

Choose the levels to collect

The filters of the logging configuration apply to the provider. AddLogging and the hosts set the minimum level to Information, so the Debug and Trace entries are not collected unless you lower it with SetMinimumLevel or AddFilter in the logging builder.

Test an ASP.NET Core application

With a WebApplicationFactory (from the Microsoft.AspNetCore.Mvc.Testing package), register the provider as a singleton ILoggerProvider in the logging builder of the application:

using var loggerProvider = new InMemoryLoggerProvider();
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);

        builder.Services.AddSingleton<ILoggerProvider>(loggerProvider);
    });
});

The application runs in the test process, so its log calls add entries to loggerProvider.Logs while the requests of the test run. To also see the logs in the output of the test, add the xUnit.net logger to the same logging builder.