Conditional formatting overwrites the format when no format is set

[ndench]

This is:

- [x] a bug report
- [ ] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

What is the expected behavior?

Conditional formats should not be changed when reading then writing an excel file.

What is the current behavior?

Reading an excel file that contains a conditional formatting rule, where the format is No format set, and then writing that file immediately results in the rule being changed to have a format.

Having No format set is a valid rule because it's the easiest way to not show any conditional formatting when the cell is blank.
ie. Have a rule for value > 0, another for value < 0, then have a No format set for value is blank that has stop if true enabled.
See "Method 1" on this post.

What are the steps to reproduce?

<?php

require __DIR__ . '/vendor/autoload.php';

// Read the excel file that has a single formatted cell
$reader = new \PhpOffice\PhpSpreadsheet\Reader\Xlsx();
$excel = $reader->load('no-format.xlsx');

// Immediately write the excel file back out without making any changes
$writer   = new \PhpOffice\PhpSpreadsheet\Writer\Xlsx($excel);
$writer->save('no-format-output.xlsx');

Here is the original file that has no content, and only a a single conditional formatting rule for A1 contains a blank value with the format Not format set
no-format.xlsx

Here is the output file after running the above script, you can see the conditional formatting rule now has a format of light red fill, dark red text.
no-format-output.xlsx

What features do you think are causing the issue

• Reader
• Writer
• Styles
• Data Validations
• Formula Calculations
• Charts
• AutoFilter
• Form Elements

Does an issue affect all spreadsheet file formats? If not, which formats are affected?

• XLSX

Which versions of PhpSpreadsheet and PHP are affected?

• PHP 8.1.2
• PhpSpreadsheet 1.25.2 (1.23.0 also affected)

Anything else?

I love PhpSpreadsheet and I'm more than happy to help debug this further/fix the issue if someone can point me in the right direction. I've tried stepping through with xdebug but it's kinda hard to follow and locate the issue.

[MarkBaker]

This is new for me, I hadn't realised that this was possible in Excel.
When we create a Conditional Formatting Rule, we always create a Style object, whether when loading a file, or when creating a CF Rule within PHP Code; and the Writers expect that Style object when saving the file.

Clearly this is a bug; and the Style should be nullable. It's a fix that will have to be done in several small steps; the internal representation will have to allow for nullable style, and all of the Writers as step #1, with the Readers as step #2, and these can be done individually - so it will likely be at least 2 releases before we can implement this fully.

[ndench]

Thanks for the response @MarkBaker. Are these changes something I can help with?