IFileSystem

IFileSystem abstracts away every I/O type from System.IO so you can inject and replace it. The interface is API-compatible with TestableIO.System.IO.Abstractions: production code written against one library works with the other.

How the BCL maps onto the interface

Static / new in System.IO	IFileSystem member
File.ReadAllText("a.txt")	fileSystem.File.ReadAllText("a.txt")
Directory.CreateDirectory("foo")	fileSystem.Directory.CreateDirectory("foo")
Path.Combine("a", "b")	fileSystem.Path.Combine("a", "b")
new FileInfo("a.txt")	fileSystem.FileInfo.New("a.txt")
new DirectoryInfo("foo")	fileSystem.DirectoryInfo.New("foo")
new FileStream(...)	fileSystem.FileStream.New(...)
new FileSystemWatcher(...)	fileSystem.FileSystemWatcher.New(...)
DriveInfo.GetDrives()	fileSystem.DriveInfo.GetDrives()
FileVersionInfo.GetVersionInfo(...)	fileSystem.FileVersionInfo.GetVersionInfo(...)

Static methods stay static on the sub-properties; constructors become factory methods (New). Everything else mirrors the framework one-to-one.

Implementations

Type	Package	Use it for
RealFileSystem	Testably.Abstractions	Production
MockFileSystem	Testably.Abstractions.Testing	Unit tests

MockFileSystem is the in-memory implementation of IFileSystem shipped in Testably.Abstractions.Testing. It implements every member of IFileSystem and is exercised by the same test suite as RealFileSystem - if a test passes against the real file system, it passes against the mock, and vice versa.

IFileSystem fileSystem = new MockFileSystem();

fileSystem.File.WriteAllText("hello.txt", "world");
string content = fileSystem.File.ReadAllText("hello.txt"); // "world"

Configuring the mock

MockFileSystem accepts an options builder (MockFileSystemOptions):

MockFileSystem fileSystem = new(o => o
    .UseCurrentDirectory("/work")                      // sets CurrentDirectory to "/work"
    .UseTimeSystem(timeSystem)                         // share an existing MockTimeSystem
    .UseRandomProvider(randomProvider)                 // share an existing IRandomProvider
    .SimulatingOperatingSystem(SimulationMode.Linux)); // simulate Linux semantics

UseCurrentDirectory() (no argument) seeds the mock with the host's actual working directory, useful when test code uses relative paths and you want them to resolve against the same place the real system would.

The mock also exposes MockFileSystem.TimeSystem and MockFileSystem.RandomSystem if you didn't wire them in yourself - use them to assert against the same instances the file system is using internally.

The remaining pages in this section cover how to seed it, simulate other operating systems, model drives, react to events, gather statistics and plug in custom strategies for ACLs, Unix file modes, safe handles and version info.