A complete example

When an exhibition closes, the gallery moves its artworks into the vault. The ExhibitionArchiver below does that, and depends on all three abstractions:

IFileSystem to move the exhibition folder and write a note,
ITimeSystem to decide whether the exhibition has closed,
IRandomSystem to generate the archive number.

ExhibitionArchiver.cs
public record Exhibition(string Folder, DateTime ClosesAt);

public class ExhibitionArchiver(
    IFileSystem fileSystem, ITimeSystem timeSystem, IRandomSystem randomSystem)
{
    public string? ArchiveIfClosed(Exhibition exhibition)
    {
        if (timeSystem.DateTime.UtcNow < exhibition.ClosesAt)
        {
            return null;
        }

        string archiveNumber = randomSystem.Guid.NewGuid().ToString("N").Substring(0, 8);
        string target = fileSystem.Path.Combine("vault", archiveNumber);

        fileSystem.Directory.CreateDirectory("vault");
        fileSystem.Directory.Move(exhibition.Folder, target);
        fileSystem.File.WriteAllText(fileSystem.Path.Combine(target, "archived-at.txt"),
            timeSystem.DateTime.UtcNow.ToString("O"));
        return archiveNumber;
    }
}

In production, register RealFileSystem, RealTimeSystem and RealRandomSystem as shown in Getting started.

The tests

The test class creates one mock of each system. The time system starts on the closing day of the exhibition, the random system returns a fixed Guid, and the file system contains the exhibition with two artworks:

ExhibitionArchiverTests.cs
public class ExhibitionArchiverTests
{
    private static readonly DateTime ClosingDay = new(2026, 4, 15, 18, 0, 0, DateTimeKind.Utc);

    private readonly MockFileSystem _fileSystem;
    private readonly MockTimeSystem _timeSystem;
    private readonly ExhibitionArchiver _sut;
    private readonly Exhibition _exhibition = new("squares", ClosingDay);

    public ExhibitionArchiverTests()
    {
        _timeSystem = new MockTimeSystem(ClosingDay.AddHours(-1));
        _fileSystem = new MockFileSystem(o => o.UseTimeSystem(_timeSystem));
        _fileSystem.Initialize()
            .WithSubdirectory("squares").Initialized(d => d
                .WithFile("red-square.png")
                .WithFile("yellow-square.png"));

        MockRandomSystem randomSystem = new(RandomProvider.Generate(
            guidGenerator: Guid.Parse("a1b2c3d4-0000-0000-0000-000000000000")));
        _sut = new ExhibitionArchiver(_fileSystem, _timeSystem, randomSystem);
    }

    [Test]
    public async Task ArchiveIfClosed_WhileOpen_ShouldNotTouchTheFileSystem()
    {
        string? archiveNumber = _sut.ArchiveIfClosed(_exhibition);

        await That(archiveNumber).IsNull();
        await That(_fileSystem.Statistics.TotalCount).IsEqualTo(0)
            .Because("seeding with Initialize() is not recorded in the statistics");
    }

    [Test]
    public async Task ArchiveIfClosed_AfterClosing_ShouldMoveTheArtworksIntoTheVault()
    {
        _timeSystem.TimeProvider.AdvanceBy(TimeSpan.FromHours(2));

        string? archiveNumber = _sut.ArchiveIfClosed(_exhibition);

        await That(archiveNumber).IsEqualTo("a1b2c3d4");
        await That(_fileSystem.Directory.Exists("squares")).IsFalse();
        await That(_fileSystem.Directory.GetFiles("vault/a1b2c3d4").Select(Path.GetFileName))
            .IsEqualTo(new[] { "archived-at.txt", "red-square.png", "yellow-square.png" }).InAnyOrder();
        await That(_fileSystem.File.ReadAllText("vault/a1b2c3d4/archived-at.txt"))
            .IsEqualTo("2026-04-15T19:00:00.0000000Z");
    }

    [Test]
    public async Task ArchiveIfClosed_WhenTheVaultIsFull_ShouldKeepTheExhibition()
    {
        _timeSystem.TimeProvider.AdvanceBy(TimeSpan.FromHours(2));
        _fileSystem.Intercept.Creating(FileSystemTypes.Directory,
            _ => throw new IOException("The vault is full."));

        void Act() => _sut.ArchiveIfClosed(_exhibition);

        await That(Act).Throws<IOException>().WithMessage("The vault is full.");
        await That(_fileSystem.Directory.GetFiles("squares")).HasCount(2);
    }
}

What each mock contributes:

Time system: the test starts an hour before closing and moves the clock with AdvanceBy, instead of waiting or depending on today's date. The MockFileSystem shares the same clock via UseTimeSystem, so file timestamps match the simulated time.
Random system: the fixed Guid makes the archive number predictable, so the test can assert the exact target folder.
File system: Initialize() seeds the exhibition, Statistics proves that nothing happened while the exhibition was open, and Intercept simulates a full vault without filling a real disk.

The tests​

The tests