Meziantou.Framework

Overview

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The Windows Projected File System (ProjFS) shows files and folders in a regular folder of the disk while their content lives elsewhere: in a database, an archive, a remote service, or memory. File Explorer and every other application see normal files. When an application lists a folder or reads a file for the first time, Windows asks your application, the provider, for the entries or the content, and keeps a local copy of what it receives.

The Meziantou.Framework.Win32.ProjectedFileSystem package implements the provider side: you derive from ProjectedFileSystemBase, return the entries of the folders and the content of the files, and the base class handles the ProjFS callbacks.

dotnet package add Meziantou.Framework.Win32.ProjectedFileSystem

The types are in the Meziantou.Framework.Win32.ProjectedFileSystem namespace.

Requirements

  • Windows 10 version 1809 or later, with the optional feature "Projected File System" installed. Install it from an elevated PowerShell prompt:

    Enable-WindowsOptionalFeature -Online -FeatureName Client-ProjFS -NoRestart

    Without the feature, Start throws a NotSupportedException.

  • A 64-bit process. The constructor of ProjectedFileSystemBase throws a NotSupportedException in a 32-bit process.

  • The provider does not need administrator rights.

How it works

  1. You create an empty folder, the root, and start the file system on it.
  2. When an application lists a folder under the root, Windows calls GetEntriesAsync of your class with the path of the folder, and shows the entries it returns.
  3. When an application accesses a file, Windows creates a placeholder: a local file with the name and the size of the entry, but no content.
  4. When an application reads the file, Windows calls OpenReadAsync and writes the content to the placeholder, which becomes hydrated. The next reads use the local copy, without calling your class.
  5. When an application writes to a file, it becomes a full file that ProjFS no longer manages: your class is not asked for its content anymore.

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