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Extensibility

IFileSystemExtensibility is the hook that custom strategies (ACL, Unix file mode) use to attach extra data to the mock without bloating the public IFileSystem surface.

note

You only need IFileSystemExtensibility when you write a custom strategy or extension method. Day-to-day test code never touches it.

public interface IFileSystemExtensibility
{
    T? RetrieveMetadata<T>(string key);
    void StoreMetadata<T>(string key, T? value);
    bool TryGetWrappedInstance<T>(out T? wrappedInstance);
}

Any IFileSystemInfo (or FileSystemStream) on the mock can be cast to IFileSystemExtensibility to:

  • Store arbitrary metadata keyed by a string. Survives for the lifetime of the file/directory entry.
  • Retrieve it back later in a strategy callback or another part of the test.
  • Unwrap the underlying real instance when the abstraction happens to point at the real file system. TryGetWrappedInstance<FileInfo>(out FileInfo? fileInfo) returns true and the actual System.IO.FileInfo only on RealFileSystem.

A typical pattern in a strategy:

public bool IsAccessGranted(string fullPath,
    IFileSystemExtensibility extensibility,
    UnixFileMode mode, FileAccess requestedAccess)
{
    string? owner = extensibility.RetrieveMetadata<string>("owner");
    return CheckPermissions(mode, owner, requestedAccess);
}

public void OnSetUnixFileMode(string fullPath,
    IFileSystemExtensibility extensibility, UnixFileMode mode)
{
    extensibility.StoreMetadata("mode", mode);
    extensibility.StoreMetadata("owner", CurrentUser);
}