disk-usage-win

A lightweight Windows disk-space GUI that scans a folder tree and shows which folders use the most space.

The executable is disk-usage-win.exe; the window title remains Fast Disk Usage. The application uses built-in Windows Forms and .NET Framework components, with no third-party packages, installer, or NuGet dependencies.

Requirements

• Windows 11
• The .NET Framework 4.x runtime to run the application.
• To build or run the regression tests, the Microsoft .NET Framework 4.x C# compiler, normally located at C:\Windows\Microsoft.NET\Framework64\v4.0.30319\csc.exe (build.cmd also checks the 32-bit Framework path)

Build

1. Extract or clone the project to a local folder.
2. Double-click build.cmd, or run it from Command Prompt.
3. The script creates disk-usage-win.exe in the project folder.

No downloads, Visual Studio, NuGet, Git, Python, .NET SDK, or third-party DLLs are required to build once the source and Framework compiler are available. Administrator rights are not required.

Run

Double-click disk-usage-win.exe, or run run.cmd.

Enter a folder path or use Browse..., then click Scan (or press Enter in the path field). While scanning, the status bar displays an animated progress indicator and live directory, file, byte, and error counts. The folder tree, folder list, and treemap populate during the scan, with updates roughly every half second. You can browse discovered folders and use Up while scanning. Sizes and percentages reflect only data discovered so far; unfinished folders are marked partial until their scan finishes. The UI remains responsive, and scanning can be canceled. Cancellation keeps the partial results visible. Finishing a scan preserves the folder you are viewing.

Select a folder in the tree, double-click a folder in the list or treemap to drill into it, and use Up to return to its parent. Starting a new scan clears the previous results and error details.

Features and performance

• Uses native Win32 FindFirstFileEx / FindNextFile enumeration.
• Uses FIND_FIRST_EX_LARGE_FETCH where supported.
• Runs scans on a background thread to keep the GUI responsive.
• Stores directory-level scan data and a separate UI-owned directory model while scanning; individual files are not retained. Updates are coalesced so a busy UI does not accumulate a queue of redraws.
• Does not traverse directory reparse points, including junctions; these appear as skipped folders to avoid loops and unexpected traversal.
• Materializes TreeView children lazily.
• Performs no content hashing, thumbnail generation, or per-file visualization.
• Renders the treemap from discovered directory totals, including provisional sizes during a scan, without additional disk I/O.
• Shows a clickable Errors: N status item. Selecting it opens a non-modal error details window with the Windows error code, operation, message, and path. The window offers Refresh, Copy All, and Save Log controls.
• Retains up to 2,000 error details to keep scan memory use predictable; the total error count is still tracked.
• Provides a right-click Open in File Explorer action for folders in the tree and folder list. Right-clicking a folder selects it first. The synthetic [files directly in this folder] row has no folder target and no Explorer action.
• Includes a WinDirStat-style treemap below the folder list. The current folder's immediate subfolders are shown as rectangles proportional to their size, with files stored directly in the current folder shown as a separate block. Double-click a folder rectangle to drill into it; right-click one to open its folder in File Explorer. Hover over a rectangle to see its path and formatted size.
• Implements the treemap with built-in WinForms and System.Drawing; it does not retain file objects or perform a second disk scan.

Interpreting results

• Sizes are logical file lengths reported by Windows, not allocated "size on disk" or drive free-space measurements. Compression, sparse files, and hard links can make these totals differ from actual storage usage.
• Individual files are not listed. Their sizes are combined into the [files directly in this folder] row and treemap block.
• Live sizes and percentages are provisional. A folder marked partial has not finished scanning; a canceled scan is not a complete inventory.
• Folders that cannot be fully enumerated are marked partial / denied in the folder list. Inspect Errors: N for details: scan completion does not imply that every folder was accessible.
• Access is limited to the current user's permissions. The application does not elevate or bypass access restrictions.

Corporate-environment notes

• No installer or elevation request.
• No telemetry, downloads, or registry writes. Scanning a network folder still requires normal filesystem access to that location.
• No service, scheduled task, driver, shell extension, or automatic persistence.
• Error logs are written only when you choose Save Log.
• No raw-disk or NTFS MFT access.
• The source is a single Program.cs file and can be reviewed before compilation.

Tests

Run test.cmd to compile and run the dependency-free scanner and WinForms regression tests using the same Framework compiler lookup as build.cmd. The tests open WinForms windows, so run them in an interactive Windows session.

Coverage includes live results before completion, final totals, snapshot isolation, coalesced updates, navigation, selection preservation, cancellation, concurrent worker/UI updates, newly discovered children in collapsed tree branches, empty folders, and error reporting for a missing folder. Temporary fixtures and the test executable are removed afterward.

Source layout

• Program.cs: application startup, native scanner, live result snapshots, WinForms UI, treemap, and error details.
• app.manifest: requests normal user privileges and enables DPI awareness.
• build.cmd: compiles disk-usage-win.exe.
• run.cmd: launches the executable; asks you to build first if it is missing.
• Tests.cs and test.cmd: regression tests and their compile/run script.

Version notes

Current changes

• Renamed the build output and launcher target to disk-usage-win.exe; application window titles and the internal namespace remain unchanged.
• Added live tree, folder-list, and treemap updates during scanning, with provisional totals and navigation before completion.
• Coalesced worker snapshots into a UI-owned directory model; finishing a scan preserves navigation, and cancellation retains partial results.
• Added scanner and WinForms regression tests, including concurrent updates and discovery of children while a tree branch is collapsed.

Earlier changes

• v2: Fixed a startup failure caused by assigning the split-pane divider before the form had its final width. Layout is deferred until the window is visible, and startup failures display an error dialog instead of failing silently.
• v3: Fixed a WinForms startup crash caused by SplitContainer minimum-size validation before the initial layout pass. No splitter minimum sizes are assigned during construction.
• v4: Added the clickable error-count status item and non-modal error details window. Error details are capped at 2,000 retained entries while the total count continues to be tracked.
• v5: Added the folder context menu with Open in File Explorer for folders in the tree and folder list.
• v6: Added the treemap for the current folder's immediate subfolders and directly stored files, with drill-down, Explorer context action, and path/size hover details.

License

MIT.