Free Bulk File Renamer for Windows — Portable, No Install

RenameZip is a portable bulk file renamer for Windows — the first desktop app in this series, and yes, it was built and compiled on a Linux server (cross-targeted with .NET 8; the download is a native Windows PE executable).

Point it at a folder and stack rules: find & replace (case-insensitive toggle), prefix and suffix, sequential numbering with configurable start, zero-padding and position (start or end of the name), and case conversion (lower / UPPER / Title). The preview updates live as you type — green rows will change, red rows are conflicts (two files mapping to the same name, or a rename that would collide with an existing file), and the Rename button stays disabled until every conflict is resolved. Extensions are always preserved, Windows-forbidden characters are stripped automatically, and one click undoes the entire last batch.

Portable in the strict sense: a single self-contained .exe — no installer, no .NET runtime to install, no registry writes, no settings files, no network access. The zip also includes the full C# source, with the rename engine in its own UI-free file.

1. Unzip anywhere — desktop, USB stick, tools folder.

2. Double-click RenameZip.exe. If Windows SmartScreen appears (unsigned indie exe), click "More info" → "Run anyway".

3. Browse to a folder, set your rules, watch the live preview, hit "Rename files". "Undo last rename" restores every name from the last batch if you change your mind.

Nothing is installed and nothing is written outside the folder you rename.

Paste this prompt into Claude to build this exact product:

Build a portable Windows bulk file renamer called RenameZip in C# / .NET 8 WinForms. Two source files: a pure, UI-free rename engine and the form. It must cross-compile from Linux (EnableWindowsTargeting) and publish as a single self-contained win-x64 exe (PublishSingleFile + EnableCompressionInSingleFile), so end users need no .NET runtime.

The engine (RenameEngine.cs, fully unit-testable without a UI): takes a list of file names plus a rules record — find/replace (ordinal, optional ignore-case), prefix, suffix, sequential numbering (start, zero-pad digits 1–8, at the start or end of the stem, joined with a dash), case mode (keep/lower/UPPER/Title with a hand-rolled title-caser) — and returns a rename plan of (oldName, newName, conflict). Extensions are always preserved; the stem is sanitized against a FIXED list of Windows-forbidden characters (not Path.GetInvalidFileNameChars, which is platform-dependent — the fixed list keeps behavior identical everywhere and makes the engine testable on Linux CI); empty results become "unnamed". Conflict = two plan entries mapping to the same target (case-insensitive) OR a rename landing on another file's current name.

The UI (single window, Segoe UI, ~860×560): folder picker; rule inputs that re-run the preview on every keystroke; a two-column ListView (current name → new name) with green rows for changes and firebrick rows for conflicts; a status line ("143 files · 89 will change · 2 CONFLICTS"); the Rename button disabled while conflicts exist; a confirmation dialog; File.Move application with per-file error tolerance; and an "Undo last rename" button that replays the last batch in reverse. Skip subfolders and dot-files. No settings persistence, no network, no installer.

Deliver RenameEngine.cs, Program.cs and the csproj. MIT license headers.

What happened when we ran it: Claude wrote the app (327 lines across the three files) and — because the engine is UI-free — we unit-tested it on the Linux build server itself: replace with case-insensitivity and extension preservation, prefix+suffix+numbering with padding, numbering at the start, lower/Title case, sanitization of every forbidden character, the no-conflict case, duplicate-target detection and collision-with-existing-file detection all passed. Two small revisions: var a = 0, b = 0; is not legal C# (changed to int), and the sanitizer originally used Path.GetInvalidFileNameChars, which on Linux only strips / — replaced with the fixed Windows list, which is also the more correct product behavior. Cross-compilation itself had a story: Ubuntu's own dotnet-sdk package ships without the WindowsDesktop targets, so the build uses Microsoft's official SDK tarball. The published artifact verifies as PE32+ executable (GUI) x86-64, for MS Windows.

Note: the download is the full, unrestricted app plus its complete source. The prompt is for reproducing (or remixing) the build yourself with your own Claude subscription.