fix: template rendering corrupts workbooks that contain formulas (calcChain + $= cell serialization)

[martin-vitous]

Problem

Rendering a template that contains any formula produces a file that Excel refuses to open ("the file is corrupt and cannot be opened" — repair also fails). Two independent defects in SaveAsByTemplate, isolated against Excel (Microsoft 365):

1. Broken calcChain.xml after rendering

A template containing formulas carries xl/calcChain.xml, whose <c> entries point at formula-cell addresses. Rendering an IEnumerable inserts rows and shifts those cells, so the chain is regenerated (#491) — but the regeneration is broken in three ways:

• Wrong cell references. ProcessFormulas derives the calcChain ref from the cell's position in the row's child list (ConvertXyToCell(ci + 1, rowIndex)). In a sparse row (e.g. a label in column A and a $= formula in column F, no cells between), the formula is child Query Support Sheet Xml c without r #2 and gets recorded as column B. Excel rejects a chain pointing at non-formula cells.
• Schema-invalid empty chain. Only $= cells are collected. When a template's formulas are all static Excel formulas (a footer VLOOKUP, a =E27*B27 row total, …), the regenerated chain has zero entries — and a calcChain with no <c> children is schema-invalid (ECMA-376), so Excel rejects the whole package.
• Stale refs accumulate. _calcChainCellRefs is never cleared between sheets, so a multi-sheet template duplicates sheet 1's refs into sheet 2's entries; and one code path copied the template's original calcChain into the output verbatim, whose refs are stale the moment rows shift.

2. $= formulas serialize as invalid cells

ProcessFormulas creates the f element without a namespace prefix; it then relies on a default xmlns declaration that CleanXml strips, leaving <f> in no namespace. The cell also keeps its t="inlineStr" attribute while no longer containing an <is> child:

<x:c r="F14" s="0" t="inlineStr"><f>SUM(G11:G13)</f></x:c>   <!-- rejected by Excel -->

Either defect alone is enough for Excel to refuse the file, so in 1.45.0 the template-formula feature and any formula-bearing template are unusable. Likely the root cause behind reports like #632/#857; reproduced identically on 2.0.0-preview.4.

Fix

ExcelOpenXmlTemplate.Impl.cs — ProcessFormulas:

• create the f element with the document's prefix so it survives CleanXml (<x:f>…</x:f>);
• remove the cell's t attribute (the cell no longer holds an inline string);
• take the calcChain ref's column from the cell's own r attribute instead of its child-list position (falls back to the old computation when r is absent).

ExcelOpenXmlTemplate.cs — SaveAsByTemplate:

• clear _calcChainCellRefs after each sheet;
• write the regenerated calcChain only when it has entries; otherwise drop the part entirely — the calculation chain is a rebuildable cache that Excel regenerates on open, while an empty or stale one makes the file unopenable;
• remove the branch that copied the template's original (stale) calcChain into the output.

Validation

• New regression tests (MiniExcelTemplateCalcChainTests) fail on current 1.45.0 sources and pass with the fix. Templates are authored in-test with ClosedXML (already a test dependency).
• Full MiniExcelTests suite: 360/360 pass.
• Manually verified with Excel (COM automation) on a matrix of templates — plain, number formats, merged cells, hidden rows + defined names, static VLOOKUP, $=SUM, and a real-world quote template (embedded images, custom XML parts, conditional formatting, dozens of static formulas). All rendered files that previously failed now open, and the formulas compute ($=SUM ranges resolve to the expanded rows, per-row static formulas recalculate).

Notes

• When the chain is dropped, the workbook relationship and [Content_Types] override still reference the absent part. Excel tolerates this; strict consumers (System.IO.Packaging) do not. Cleaning those two entries requires touching the entry-copy path — happy to add it to this PR if you prefer.
• Unrelated pre-existing quirk (not addressed here): defined names whose range lies below the inserted rows are not shifted, so formulas referencing them can mis-resolve after rendering.
• The same two defects reproduce on the 2.0.0-preview line; I can port this fix there if wanted.

Summary by CodeRabbit

• Bug Fixes
  • Fixed formula cells generated from templates so they retain correct Excel formatting and cell references, including sparse rows.
  • Prevented invalid inline-string formula cells from being created.
  • Corrected calculation-chain entries after template rendering, avoiding stale or empty formula references.

[coderabbitai]

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[coderabbitai]

Actionable comments posted: 1

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

src/MiniExcel/SaveByTemplate/ExcelOpenXmlTemplate.cs (1)

135-152: 🗄️ Data Integrity & Integration | 🟠 Major | ⚡ Quick win

Remove calcChain package metadata when omitting the part.

When _calcChainContent is empty, this drops xl/calcChain.xml, but the copied xl/_rels/workbook.xml.rels still targets it and [Content_Types].xml still declares it. That leaves an invalid package relationship. Remove the calcChain relationship and its content-type override whenever no calcChain part is emitted; extend TemplateWithStaticFormula_DoesNotWriteStaleOrEmptyCalcChain to assert both are absent.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/MiniExcel/SaveByTemplate/ExcelOpenXmlTemplate.cs` around lines 135 - 152,
Update the calcChain handling around _calcChainContent and
CalcChainHelper.GenerateCalcChainSheet so that when no calcChain content is
emitted, the corresponding calcChain relationship in workbook.xml.rels and its
[Content_Types].xml override are also removed. Preserve generation of the part
and metadata when content exists, and extend
TemplateWithStaticFormula_DoesNotWriteStaleOrEmptyCalcChain to assert both
metadata entries are absent.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/MiniExcel/SaveByTemplate/ExcelOpenXmlTemplate.Impl.cs`:
- Around line 1018-1023: Update the formula-processing logic around celRef to
use the complete cell reference from c.GetAttribute("r") directly, preserving
the existing fallback to ExcelOpenXmlUtils.ConvertXyToCell only when the r
attribute is absent. Ensure calcChain entries use the rendered address rather
than the advanced newRowIndex, and add a regression case covering $= inside an
expanded enumerable row.

---

Outside diff comments:
In `@src/MiniExcel/SaveByTemplate/ExcelOpenXmlTemplate.cs`:
- Around line 135-152: Update the calcChain handling around _calcChainContent
and CalcChainHelper.GenerateCalcChainSheet so that when no calcChain content is
emitted, the corresponding calcChain relationship in workbook.xml.rels and its
[Content_Types].xml override are also removed. Preserve generation of the part
and metadata when content exists, and extend
TemplateWithStaticFormula_DoesNotWriteStaleOrEmptyCalcChain to assert both
metadata entries are absent.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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Reviewing files that changed from the base of the PR and between a32b538 and 4e74b72.

📒 Files selected for processing (3)

• src/MiniExcel/SaveByTemplate/ExcelOpenXmlTemplate.Impl.cs
• src/MiniExcel/SaveByTemplate/ExcelOpenXmlTemplate.cs
• tests/MiniExcelTests/SaveByTemplate/MiniExcelTemplateCalcChainTests.cs

[michelebastione]

@martin-vitous Thank you for your contribution, I'm reviewing it now. If you could please prepare a PR for the main branch it would be appreciated.