Testing

Testing with Mocked File System

ModularPipelines supports mocking file system operations for unit testing. All file I/O goes through IFileSystemProvider, which can be replaced with a mock implementation.

Why Mock the File System?

• Speed: Tests run faster without actual disk I/O
• Isolation: Tests don't depend on file system state
• Predictability: No flaky tests due to file permissions or disk space
• CI-friendly: Works in any environment without file system setup

Example: Mocking File Reads

using Moq;
using ModularPipelines;
using ModularPipelines.FileSystem;
using ModularPipelines.Extensions;

[Test]
public async Task MyModule_ReadsConfigFile()
{
    // Create a mock provider
    var mockProvider = new Mock<IFileSystemProvider>();
    mockProvider.Setup(p => p.ReadAllTextAsync(
            It.IsAny<string>(),
            It.IsAny<CancellationToken>()))
        .ReturnsAsync("{\"setting\": \"value\"}");

    // Run pipeline with mock
    var builder = Pipeline.CreateBuilder(args);

    builder.Services.AddSingleton<IFileSystemProvider>(mockProvider.Object);
    builder.Services.AddModule<MyModule>();

    var result = await builder.Build().RunAsync();

    // Assert results
    Assert.That(result.Status, Is.EqualTo(PipelineStatus.Success));
}

Example: Verifying File Writes

[Test]
public async Task MyModule_WritesOutputFile()
{
    var mockProvider = new Mock<IFileSystemProvider>();

    var builder = Pipeline.CreateBuilder(args);

    builder.Services.AddSingleton<IFileSystemProvider>(mockProvider.Object);
    builder.Services.AddModule<OutputModule>();

    await builder.Build().RunAsync();

    // Verify the write occurred with expected content
    mockProvider.Verify(p => p.WriteAllTextAsync(
        It.Is<string>(path => path.Contains("output")),
        It.Is<string>(content => content.Contains("result")),
        It.IsAny<CancellationToken>()));
}

Important Notes

• Always use context.Files: Files created via context.Files.GetFile() will use the injected provider. Files created directly via new File("path") use the real file system.

• Provider Registration: The mock provider must be registered before the pipeline runs. Using services.AddSingleton<IFileSystemProvider>() overrides the default SystemFileSystemProvider.

• Mock ALL methods your code uses: The mock provider only intercepts methods you explicitly set up. If your module calls ReadAllTextAsync, FileExists, and Combine, you must mock all three. Unmocked methods may throw or return default values depending on your mocking framework.

• Implicit operators bypass mocking: Implicit conversions like File file = "/path/to/file" create instances using the default SystemFileSystemProvider, not your mock. For full testability, always use context.Files.GetFile().

• Static methods are not mockable: Methods like File.GetNewTemporaryFilePath() and Folder.CreateTemporaryFolder() use the real file system. Design your modules to receive paths via constructor or use context.Files.CreateTemporaryFolder() instead.

• Mocking Path Operations: If your code uses path operations, mock them too:

  mockProvider.Setup(p => p.Combine(It.IsAny<string[]>()))
      .Returns((string[] paths) => Path.Combine(paths));

What Gets Mocked

The IFileSystemProvider interface covers:

• File reads: ReadAllTextAsync, ReadLinesAsync, ReadAllBytesAsync
• File writes: WriteAllTextAsync, WriteAllBytesAsync, WriteAllLinesAsync, AppendAllTextAsync
• File management: DeleteFile, CopyFile, MoveFile, FileExists
• Directory operations: CreateDirectory, DeleteDirectory, MoveDirectory, DirectoryExists
• Enumeration: EnumerateFiles, EnumerateDirectories
• Path utilities: GetTempPath, GetRandomFileName, Combine, GetRelativePath