DefaultFast

Fast, portable Windows default app manager and file association repair utility.

About • Features • Screenshots • How It Works • Installation • Usage • Repair & Safety • Building from Source • Diagnostics & Budgets • License

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About

DefaultFast is a lightweight, portable Windows utility to instantly configure and repair default file types and protocol handlers. Windows Settings requires navigating multi-layered pages to change associations; DefaultFast lets you search an extension (.pdf), a protocol (https), or an application name (VLC), select your choice, and apply it in one keystroke.

It also automatically diagnoses and fixes broken file associations — including the recurring Windows bug where double-clicking .bat or .cmd scripts prompts "Select an app to open this file".

Every change is executed through a verified transaction: keys are backed up to standard .reg files before writing, Shell registration is queried for confirmation, and changes roll back automatically if the system refuses them. DefaultFast runs as a standard user process with zero installers, zero runtime dependencies, and no network access.

Features

• Instant unified search: Search across 900+ file extensions, 170+ protocols, and all installed desktop and packaged apps in under 3 ms.
• Batch application defaults: View every file type supported by a program (e.g. Media Player or Notepad) and associate them all or a selected subset with a single click.
• Automatic association repair: One-click health check detects empty handlers, missing ProgIDs, and shadowed extensions (such as broken .bat, .cmd, .lnk, .exe, or .ps1 associations).
• Safe transaction & rollback: Computes legitimate UserChoice hashes; every write is verified through Windows Shell API (AssocQueryStringW) before marking successful, with instant rollback on denial.
• Automatic .reg backups: Exports registry states to %LOCALAPPDATA%\DefaultFast\backups\*.reg prior to modification, enabling one-click in-app Undo or standalone manual restoration.
• Native Fluent UI & Snap integration: Built with Slint and native Win32 window chrome supporting Windows 11 Snap Layouts, system dark/light themes, and keyboard navigation (Ctrl+K, /, Enter, Esc).
• Minimal footprint: Interactive window renders in under 95 ms with an idle memory footprint of approximately 21 MB.

Screenshots

Instant search view: quick suggestions for extensions, protocols, and programs with background health scanning.

Extension detail view: inspect the active handler, see registered programs, and reassign defaults in one click.

Program detail view: bulk-assign all supported file types or selectively toggle specific formats.

Health check pane: detect broken associations and restore missing or shadowed registry handlers.

How It Works

Windows restricts default app registration by storing user choices under HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\<ext>\UserChoice protected by an undocumented hash. An invalid hash triggers an association reset or silent rejection.

DefaultFast implements a safe association transaction pipeline:

flowchart TD
    Start([User selects target handler]) --> Preflight{Preflight Checks}
    Preflight -->|Protected by UserChoiceLatest / UCPD| SettingsLink[Offer deep-link to Windows Settings]
    Preflight -->|Valid target| Backup[Export affected keys to backups/*.reg]
    
    Backup --> RegProgId[Register HKCU Applications ProgID if required]
    RegProgId --> Hash[Calculate UserChoice MD5/rotate checksum]
    Hash --> RegWrite[Write HKCU UserChoice ProgId + Hash]
    
    RegWrite --> Verify{Shell Verification AssocQueryStringW}
    Verify -->|Shell confirms new default| Notify[SHChangeNotify SHCNE_ASSOCCHANGED]
    Notify --> Toast[Success toast with Undo button]
    
    Verify -->|Shell rejects or mismatch| Rollback[Roll back from .reg backup]
    Rollback --> ErrorToast[Report failure & provide Windows Settings fallback]

Loading

1. Preflight: Checks whether the target uses Windows 11's protected UserChoiceLatest or the UCPD driver (e.g. http, https, .pdf on recent OS builds). If blocked, it directs you cleanly to the appropriate Windows Settings page without half-applied changes.
2. Registry Backup: Before altering keys, DefaultFast generates a .reg file in %LOCALAPPDATA%\DefaultFast\backups\.
3. Hash Calculation: Generates the UserChoice hash matching the algorithm verified against Windows-generated keys on your machine.
4. Shell Verification: Asserts that Explorer (AssocQueryStringW) acknowledges the new association. If verification fails, the backup is restored immediately.
5. Explorer Notification: Issues SHChangeNotify(SHCNE_ASSOCCHANGED) so icons and file handlers update instantly without restarting Explorer.

Installation

Requirements

• OS: Windows 10 (version 1809 or higher, build 17763+) or Windows 11 (x86_64).
• Permissions: Runs entirely under standard user privileges. Administrator rights (UAC) are only requested when repairing machine-level HKLM keys for core file types.
• Dependencies: None. The single executable is self-contained.

Download

Download the latest executable DefaultFast.exe from GitHub Releases.

Place DefaultFast.exe in any folder or your PATH. No installation or setup process is needed.

Usage

Keyboard Shortcuts & Navigation

Action	Shortcut / Input
Focus Search	Ctrl+K or /
Navigate Results	↑ / ↓ arrow keys
Open Detail Pane	Enter on a selected result
Toggle Checkbox	Space in the supported types list
Apply Default	Ctrl+Enter or click Set default / Set as default for selected
Make Default for All	Click Make default for everything
Dismiss / Back	Esc (detail pane → search bar → clear query → home)
Undo Last Change	Click Undo on toast notification or navigate to History

Command-Line Arguments

You can integrate DefaultFast into scripts, run diagnostics, or create shortcuts for specific workflows:

# Open directly with a search pre-filled:
.\DefaultFast.exe .pdf
.\DefaultFast.exe https
.\DefaultFast.exe Notepad

# Open directly into the Association Health Check:
.\DefaultFast.exe --repair

# Run headless diagnostics and generate diagnostics.txt:
.\DefaultFast.exe --diagnose

# Execute a self-contained registry write/verify/rollback test:
.\DefaultFast.exe --self-test

# Run cold-start and memory probe (used by measurement scripts):
.\DefaultFast.exe --startup-probe

Unregistered Portable Apps

For standalone editors or portable utilities that do not create standard registry entries, DefaultFast provides an Add type field in their detail view. Entering .txt or another extension automatically creates HKCU\Software\Classes\Applications\<exe>\shell\open\command and links SupportedTypes, making the portable binary a first-class choice in Windows.

Repair and Safety

What the Health Check Detects

DefaultFast monitors critical system file associations against a known-good configuration table (.bat, .cmd, .exe, .lnk, .msi, .ps1, .reg, .txt, .url, .vbs, .js, .zip):

• Empty or shadowed default ProgIDs (e.g. an accidental empty HKCU\Software\Classes\.bat key breaking batch files).
• Missing handler binaries or uninstalled ProgIDs assigned in UserChoice.
• Corrupted or invalid UserChoice hash signatures.

Elevation Policy

DefaultFast never runs elevated by default. When an association requires repairing HKLM\SOFTWARE\Classes (such as a damaged system .cmd entry):

1. A prompt displays the specific keys needing repair.
2. Upon confirmation, DefaultFast launches an elevated one-shot worker via runas:

   DefaultFast.exe --elevated-fix .bat .cmd

3. The elevated worker validates each extension against the internal known-good table and rejects any writes outside HKLM\SOFTWARE\Classes\.<ext>.

Data and Backup Directory

All state is stored in %LOCALAPPDATA%\DefaultFast:

• backups\*.reg — Automated backups saved prior to every registry edit (up to 50 kept).
• log.txt — Append-only audit log recording every registry modification.
• window.txt — Saved window size, position, and maximize state.

Building from Source

Prerequisites

• Rust: Version 1.88+ (stable-x86_64-pc-windows-msvc).
• Visual Studio Build Tools: Desktop development with C++ workload (MSVC linker and the Windows SDK with rc.exe).

Build Steps

# Clone the repository
git clone https://github.com/projectGloria/DefaultFast.git
cd DefaultFast

# Run test suite (31 unit tests covering hash, registry, and repairs)
cargo test

# Build optimized release binary
cargo build --release

The compiled standalone executable is generated at:

target\release\DefaultFast.exe

Diagnostics and Budgets

DefaultFast is designed to satisfy strict performance and memory budgets:

Metric	Target	Measured (Win 11 Build 26100)	Status
Startup to First Frame	< 150 ms	92–95 ms	Passed
Idle RAM (Working Set)	< 30 MB	21.7 MB (≈11 MB private)	Passed
Search & Re-render Latency	< 16 ms	1.7–3.2 ms (Search core ≈0.45 ms)	Passed
Binary Size	< 6 MB	8.28 MB (Slint 1.18 + software renderer)	Accepted trade-off

Run the benchmark script locally across multiple iterations:

.\scripts\measure.ps1 -Runs 5

See docs/TESTPLAN.md for the manual quality assurance and verification test plan.

License

This project is licensed under the MIT License.

Acknowledgments

• UserChoice hash reverse-engineering based on research by SetUserFTA and PS-SFTA.
• Graphical user interface powered by Slint.