This library is a feature complete testing helper for the IFileSystem abstractions for I/O-related functionality from the System.IO namespace. It uses an in-memory file system that behaves exactly like the real file system and can be used in unit tests for dependency injection.
The testing helper also supports advanced scenarios like

• Multiple drives with limited size
• FileSystemWatcher and
• a way to work with SafeFileHandles

The companion projects Testably.Abstractions.Compression and Testably.Abstractions.AccessControl allow working with Zip-Files and Access Control Lists respectively.

As the test suite runs both against the mocked and the real file system, the behaviour between the two is identical and it also allows simulating the file system on other operating systems (Linux, MacOS and Windows).

In addition, the following interfaces are defined:

• The ITimeSystem interface abstracts away time-related functionality:
  • DateTime methods give access to the current time
  • Task allows replacing Task.Delay
  • Thread allows replacing Thread.Sleep
  • Timer is a wrapper around System.Threading.Timer
• The IRandomSystem interface abstracts away functionality related to randomness:
  Random methods implement a thread-safe Shared instance also under .NET Framework and Guid methods allow creating new GUIDs.

Example

Use the interfaces and their default implementations using your prefered dependency injection method, e.g.:

private readonly IFileSystem _fileSystem;

public class MyService(IFileSystem fileSystem)
{
    _fileSystem = fileSystem;
}

public void StoreData()
{
    var fileContent = GetFileContent();
    _fileSystem.File.WriteAllText("result.xml", fileContent);
}

private string GetFileContent()
{
    // Generate the file content
}

Then you test your class with the mocked types in Testably.Abstractions.Testing:

[Fact]
public void StoreData_ShouldWriteValidFile()
{
    IFileSystem fileSystem = new MockFileSystem();
    MyService sut = new MyService(fileSystem);

    sut.StoreData();

    var fileContent = fileSystem.File.ReadAllText("result.xml");
    // Validate fileContent
}

More examples can be found in the examples section!

Getting Started

• Install Testably.Abstractions as nuget package in your production projects and Testably.Abstractions.Testing as nuget package in your test projects.

  dotnet add package Testably.Abstractions
  dotnet add package Testably.Abstractions.Testing

• Configure your dependeny injection framework, e.g. with Microsoft.Extensions.DependencyInjections in ASP.NET core:

  builder.Services
      .AddSingleton<IFileSystem, RealFileSystem>()
      .AddSingleton<IRandomSystem, RealRandomSystem>()
      .AddSingleton<ITimeSystem, RealTimeSystem>();

You can now use the interfaces in your services!

Testing

In order to simplify testing, the Testably.Abstractions.Testing project provides mocked instances for the abstraction interfaces, which are configured using fluent syntax:

Initialization

The following two code snippets initialize the mocked fileSystem with a structure like the following:

• Directory "foo"
  • Directory "bar"
  • Empty file "bar.txt"
• File "foo.txt" with "some file content" as content

var fileSystem = new MockFileSystem();
fileSystem.Initialize().With(
    new DirectoryDescription("foo",
        new DirectoryDescription("bar"),
        new FileDescription("bar.txt")),
    new FileDescription("foo.txt", "some file content"));

var fileSystem = new MockFileSystem();
fileSystem.Initialize()
	.WithSubdirectory("foo").Initialized(d => d
		.WithSubdirectory("bar")
		.WithFile("bar.txt"))
	.WithFile("foo.txt").Which(f => f.HasStringContent("some file content"));

Simulating other operating systems

The MockFileSystem can also simulate other operating systems than the one it is currently running on. This can be achieved, by providing the corresponding SimulationMode in the constructor:

var linuxFileSystem = new MockFileSystem(o => o.SimulatingOperatingSystem(SimulationMode.Linux));
// The `linuxFileSystem` now behaves like a Linux file system even under Windows:
// - case-sensitive
// - slash as directory separator

var windowsFileSystem = new MockFileSystem(o => o.SimulatingOperatingSystem(SimulationMode.Windows));
// The `windowsFileSystem` now behaves like a Windows file system even under Linux or MacOS:
// - multiple drives
// - case-insensitive
// - backslash as directory separator

By running all tests against the real file system and the simulated under Linux, MacOS and Windows, the behaviour is consistent between the native and simulated mock file systems.

Drive management

var fileSystem = new MockFileSystem();
fileSystem
    .WithDrive("D:", d => d
        .SetTotalSize(1024 * 1024))
    .InitializeIn("D:")
    .WithFile("foo.txt")
    .WithSubdirectory("sub-dir").Initialized(s => s
        .WithAFile(".json").Which(
            f => f.HasStringContent("{\"count\":1}")));

Initializes the mocked file system with a second drive D: with 1MB total available space and creates on it an empty text file foo.txt and a directory sub-dir which contains randomly named json file with {"count":1} as file content.

On non-Windows systems, the main drive can still be configured, e.g.

var fileSystem = new MockFileSystem();
fileSystem.WithDrive(d => d.SetTotalSize(20));

// this will throw an IOException that there is not enough space on the disk.
fileSystem.File.WriteAllText("foo", "some text longer than 20 bytes");