Decompiling an Excel template into source - #1 C#

Kookerella Ltd. released a .NET 10 file-based app and CLI tool, fsopenxmldsl-mcp, that decompiles Excel templates into C# source code, enabling developers to reproduce spreadsheet styling programmatically. The tool, available as a dotnet global tool, converts an .xlsx file into a runnable .cs file that regenerates the original spreadsheet pixel-for-pixel, including styles and formulas, and the company demonstrated the workflow in a blog post with a sample invoice template.

Decompiling an Excel template into source - 1 C You don’t have to hand-write Excel styling code. If someone already built the spreadsheet you need to reproduce — a finance person’s invoice template, a report layout a designer signed off on — you can reverse-engineer it into real source code, then wire in your own data. This is the first of four posts walking through the same demo in C , F , XSLT, and plain JSON. The full project is on GitHub: Kookerella.Demo.DecompileToSource https://github.com/Kookerella-Ltd/Kookerella.Demo.DecompileToSource . What we’re starting from A plain Excel file, built by hand in Excel. It has: - A bold, 16pt “INVOICE” title - A header row styled white-on-navy with a bottom border - Two sample line-item rows, with an Amount column computed as Qty Unit Price Nobody wrote any code to produce this file — it’s just a spreadsheet. Step 1: Install the tool The reverse-engineering happens via a CLI tool, fsopenxmldsl-mcp , published as a dotnet global tool: dotnet tool install -g Kookerella.FsOpenXmlDsl.Mcp Step 2: Reverse-engineer the template fsopenxmldsl-mcp convert templates/InvoiceTemplate.xlsx --lang csharp -o decompiled/InvoiceTemplate.g.cs This reads the real .xlsx file and prints out C code that reproduces it exactly. The top line of the result is: :package Kookerella.CsOpenXmlDsl@0.3.2 That :package line is a .NET 10 file-based app https://learn.microsoft.com/dotnet/core/whatsnew/dotnet-10 file-based-apps directive — this single .cs file is a complete, runnable program with no .csproj needed. You can run it immediately, on any machine with the .NET 10 SDK: cd decompiled dotnet run InvoiceTemplate.g.cs That produces output.xlsx — pixel-for-pixel the same as the original template. Nothing was lost in translation: title, header styling, borders, currency formatting, and the Qty Unit Price formulas are all there, expressed as plain C method calls Cell.Text ... , .WithStyle ... , Cell.Formula ... , and so on . This is the point where “decompiling a spreadsheet” stops being a metaphor. You now have the template as source code you can read, diff, and edit. Step 3: Adding the package to a real project The file-based app above pinned the package inline for convenience. In a normal project you add it the usual way: dotnet add package Kookerella.CsOpenXmlDsl Kookerella.CsOpenXmlDsl is a C wrapper around a separate F core library, Kookerella.FsOpenXmlDsl , which does the actual OOXML reading/writing. Step 4: Turning the decompiled snippet into real code The decompiled file is disposable — it’s the starting point, not the deliverable. Nobody wants to hardcode “Widgets” and “Gadgets” forever. The diff from the decompiled file to a maintained version is deliberately small: Wrap the top-level statements in a class and a Build method that takes real data IEnumerable<OrderLine instead of running standalone.Replace the two hardcoded item rows with a LINQ projection over that data: js var dataRows = orders.Select order, i = { var row = 4 + i; return Row.Of Cell.Text order.Item , Cell.Number order.Quantity , Cell.Number order.UnitPrice .WithStyle CellStyle.Default.WithNumberFormat NumberFormatKind.Currency , Cell.Formula $"B{row} C{row}", order.Quantity order.UnitPrice .WithStyle CellStyle.Default.WithNumberFormat NumberFormatKind.Currency ; } ; Splice the generated rows into the sheet with a C collection expression, instead of listing two hardcoded rows: js var sheet0 = Sheet.Create "Invoice", / title row, exactly as decompiled /, / header row, exactly as decompiled /, .. dataRows, ; Everything else — every style call, every color, every border — is copied verbatim from the decompiled file. The styling was never hand-written by a developer in the first place, so there’s nothing to “port” — only the two data rows needed to become data-driven. Step 5: Prove it stays correct Because this is now real code, it can be tested like real code — something you can’t do to a .xlsx file sitting on a file share. The test suite checks three things a spreadsheet can’t check about itself: Schema validity — runs a real OpenXmlValidator over the output. If a future change ever produces invalid OOXML, this catches it immediately instead of someone discovering a “repair this file?” dialog in Excel weeks later. Correctness — feeds in 0, 1, and 3 orders and checks the row count and the Amount formula text/value are correct for each one. Styling preservation — pins the exact colors, bold, font size, and border style that came from the original template, so a future edit can’t silently drift the invoice’s look away from what the original designer signed off on. Recap | Step | Command | Output | |---|---|---| | Install the tool | dotnet tool install -g Kookerella.FsOpenXmlDsl.Mcp | fsopenxmldsl-mcp on your PATH | | Reverse-engineer | fsopenxmldsl-mcp convert InvoiceTemplate.xlsx --lang csharp -o InvoiceTemplate.g.cs | A runnable C file that reproduces the original .xlsx exactly | | Add the package | dotnet add package Kookerella.CsOpenXmlDsl | The C wrapper over the F core | | Make it data-driven | Replace hardcoded rows with a LINQ projection over your own data model | A maintained generator | | Verify | dotnet test | Schema validity, formula correctness, and styling all pinned by real tests | The template never had to be redesigned in code, and the developer never had to reverse-engineer OOXML XML by hand — the tool did that translation, once, and everything after that is ordinary, testable C . Next up: the same demo in F /posts/decompiling-excel-template-2-fsharp/ .