Testably.Abstractions

Testably.Abstractions provides injectable abstractions for the static parts of the .NET base class library (BCL), namely the file system, time and randomness, with feature-complete in-memory mocks for tests.

Inject. Mock. Test.

Depend on IFileSystem, ITimeSystem and IRandomSystem in production. Swap in the in-memory mocks for tests: deterministic, cross-platform, no temp folders or Thread.Sleep.

File system

CatalogService.cs

public class CatalogService(IFileSystem fileSystem)
{
    public void Export(string json)
    {
        fileSystem.Directory.CreateDirectory("exports");
        fileSystem.File.WriteAllText("exports/catalog.json", json);
    }
}

CatalogServiceTests.cs

[Test]
public async Task Export_WritesCatalogToExportsFolder()
{
    MockFileSystem fileSystem = new();
    CatalogService sut = new(fileSystem);

    sut.Export("""{ "title": "Red Square" }""");

    await That(fileSystem.File.ReadAllText("exports/catalog.json"))
        .IsEqualTo("""{ "title": "Red Square" }""");
}

Time system

Exhibition.cs

public class Exhibition(ITimeSystem timeSystem, DateTime closesAt)
{
    public bool IsOpen => timeSystem.DateTime.UtcNow < closesAt;
}

ExhibitionTests.cs

[Test]
public async Task IsOpen_ReturnsFalse_AfterClosingDate()
{
    MockTimeSystem timeSystem = new(new DateTime(2026, 3, 1, 0, 0, 0, DateTimeKind.Utc));
    Exhibition sut = new(timeSystem, closesAt: new DateTime(2026, 4, 15, 0, 0, 0, DateTimeKind.Utc));

    timeSystem.TimeProvider.AdvanceBy(TimeSpan.FromDays(60));

    await That(sut.IsOpen).IsFalse();
}

Random system

ArtworkOfTheDay.cs

public class ArtworkOfTheDay(IRandomSystem randomSystem, string[] artworks)
{
    public string Pick() => artworks[randomSystem.Random.Shared.Next(artworks.Length)];
}

ArtworkOfTheDayTests.cs

[Test]
public async Task Pick_ReturnsTheArtworkAtTheRandomIndex()
{
    MockRandomSystem randomSystem = new(RandomProvider.Generate(intGenerator: 1));
    ArtworkOfTheDay sut = new(randomSystem, ["Red Square", "Blue Circles", "Yellow Lines"]);

    await That(sut.Pick()).IsEqualTo("Blue Circles");
}

Why Testably.Abstractions?

Testably.Abstractions shares the IFileSystem interface with TestableIO.System.IO.Abstractions, so your production code does not change when you switch testing libraries. What you get on top:

• Advanced testing scenarios: FileSystemWatcher, SafeFileHandle, multiple drives with size limits.
• Cross-platform simulation: run a Linux file system on Windows (and vice versa) to exercise platform-specific behavior from a single CI job.
• A second abstraction layer for time and randomness.

See Migration from TestableIO if you are already using TestableIO.

Where to go next

• Getting started: install the packages and wire them up.
• A complete example: one gallery service tested with all three abstractions.
• File system, Time system, Random system: explore each abstraction.
• Companion libraries: ZIP, ACL and memory-mapped file helpers.
• aweXpect.Testably: expectations for the file system and time system mocks.