This library is an alternative testing helper for the IFileSystem interfaces from System.IO.Abstractions. These interfaces allow abstracting away all I/O-related functionality from the System.IO namespace.
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.

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.

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!

• Install Testably.Abstractions as nuget package in your productive 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!

In order to simplify testing, the Testably.Abstractions.Testing project provides mocked instances for the abstraction interfaces:

These mocks are configured using fluent syntax:

new MockFileSystem()
    .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.