How to assert timer callback in c# xunit unit test

[fingers10]

I have a OfflineHourlyDatabaseBackup BackgroundService in .NET 8 as shown below.

OfflineHourlyDatabaseBackup:

public class OfflineHourlyDatabaseBackup(
	ILogger<OfflineHourlyDatabaseBackup> logger,
	TimeProvider timeProvider,
	IServiceProvider serviceProvider) : BackgroundService
{
	protected override async Task ExecuteAsync(CancellationToken stoppingToken)
	{
		await Task.Yield();

		timeProvider.CreateTimer(Backup, null, TimeSpan.Zero, TimeSpan.FromHours(1));

		stoppingToken.Register(() => logger.LogWarning($"{nameof(OfflineHourlyDatabaseBackup)} is stopping due to host shut down."));
	}

	private void Backup(object? state)
	{
		using var scope = serviceProvider.CreateScope();
		var offlineDatabaseBackupService = scope.ServiceProvider.GetRequiredService<IOfflineDatabaseBackupService>();
		offlineDatabaseBackupService.Backup();
	}
}

I have written uni test to test the behavior. Here is my xunit unit test,

[Fact]
public async Task OfflineHourlyDatabaseBackup_ExecuteAsync()
{
	// Arrange
	var serviceCollection = new ServiceCollection();
	var mockOfflineDatabaseBackupService = new Mock<IOfflineDatabaseBackupService>();
	serviceCollection.AddSingleton(_ => mockOfflineDatabaseBackupService.Object);
	var now = DateTimeOffset.UtcNow;
	var timeProvider = new TestTimeProvider(now);
	var serviceProvider = serviceCollection.BuildServiceProvider();
	var logger = NullLogger<OfflineHourlyDatabaseBackup>.Instance;
	var configuration = new Mock<IConfiguration>();

	// Act
	using var offlineHourlyDatabaseBackup = new OfflineHourlyDatabaseBackup(logger, timeProvider, serviceProvider);
	await offlineHourlyDatabaseBackup.StartAsync(default);
	await offlineHourlyDatabaseBackup.ExecuteTask!; // --> Backup() should be called
	timeProvider.Advance(TimeSpan.FromHours(1)); // --> Backup() should be called

	// Assert
	offlineHourlyDatabaseBackup.ExecuteTask.IsCompletedSuccessfully.Should().BeTrue();
	mockOfflineDatabaseBackupService.Verify(x => x.Backup(), Times.AtLeast(2));
}

Here is my TestTimeProvider.cs,

internal class TestTimeProvider(DateTimeOffset now) : TimeProvider
{
	public override DateTimeOffset GetUtcNow() => now;

	public override ITimer CreateTimer(TimerCallback callback, object? state, TimeSpan dueTime, TimeSpan period)
	{
		return base.CreateTimer(callback, state, TimeSpan.Zero, TimeSpan.FromHours(1));
	}

	public void Advance(TimeSpan timeSpan)
	{
		now = now.Add(timeSpan);
	}
}

When I run the test I expect Callback Backup method to be called two times. One because of await offlineHourlyDatabaseBackup.ExecuteTask!; and another time because of timeProvider.Advance(TimeSpan.FromHours(1)); but getting the exception message as follows

Expected invocation on the mock at least 2 times, but was 0 times: x
=> x.Backup()

Upon debugging I have noticed that before the first Backup() call gets executed the assertion line gets completed and throws above error.

Please can you assist me on what I'm doing wrong here ?

[martincostello]

Have you considered using the FakeTimeProvider implementation from NuGet instead of rolling your own? https://devblogs.microsoft.com/dotnet/fake-it-til-you-make-it-to-production/

[fingers10]

@martincostello Many thanks that helped and I solved the issue.