Fix IndexOutOfRangeException when parsing markdown tables without trailing pipes

[Copilot]

Markdig parses tables without trailing | characters differently than tables with them, causing table.ColumnDefinitions.Count to vary. The code assumed all tables needed -1 adjustment for column count, which failed for tables without trailing pipes.

Example of failing markdown:

| Flag                                       |      Value | Description
|:-------------------------------------------|-----------:|:-----------
| READYTORUN_FLAG_PLATFORM_NEUTRAL_SOURCE    | 0x00000001 | Set if the original IL
| READYTORUN_FLAG_COMPOSITE                  | 0x00000002 | The image represents

Changes

• MarkdownParser.cs: Replace hardcoded table.ColumnDefinitions.Count - 1 with dynamic GetColumnCount() that examines actual row cells and their column spans
• MarkdownReaderTests.cs: Add test case with table from issue demonstrating both multi-column parsing and empty cell handling

The fix calculates column count by iterating rows and summing ColumnSpan values, returning the maximum found. This handles both table formats correctly.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>[MEDI] Parsing markdown fails for tables without trailing |</issue_title>
<issue_description>A bug reported offline by @KrystofS :

Sample markdown that breaks the reader, taken from here:

| Flag                                       |      Value | Description
|:-------------------------------------------|-----------:|:-----------
| READYTORUN_FLAG_PLATFORM_NEUTRAL_SOURCE    | 0x00000001 | Set if the original IL image was platform neutral. The platform neutrality is part of assembly name. This flag can be used to reconstruct the full original assembly name.
| READYTORUN_FLAG_COMPOSITE                  | 0x00000002 | The image represents a composite R2R file resulting from a combined compilation of a larger number of input MSIL assemblies.
| READYTORUN_FLAG_PARTIAL                    | 0x00000004 |
| READYTORUN_FLAG_NONSHARED_PINVOKE_STUBS    | 0x00000008 | PInvoke stubs compiled into image are non-shareable (no secret parameter)
| READYTORUN_FLAG_EMBEDDED_MSIL              | 0x00000010 | Input MSIL is embedded in the R2R image.
| READYTORUN_FLAG_COMPONENT                  | 0x00000020 | This is a component assembly of a composite R2R image
| READYTORUN_FLAG_MULTIMODULE_VERSION_BUBBLE | 0x00000040 | This R2R module has multiple modules within its version bubble (For versions before version 6.3, all modules are assumed to possibly have this characteristic)
| READYTORUN_FLAG_UNRELATED_R2R_CODE         | 0x00000080 | This R2R module has code in it that would not be naturally encoded into this module
| READYTORUN_FLAG_PLATFORM_NATIVE_IMAGE      | 0x00000100 | The owning composite executable is in the platform native format

fail: Microsoft.Extensions.DataIngestion.IngestionPipeline[6]
      An error occurred while ingesting document 'C:\Users\Krystof\Documents\GitHub\chat-demo\MyLocalAIApp\wwwroot\Data\readytorun-format.md'.
      System.IndexOutOfRangeException: Index was outside the bounds of the array.
         at Microsoft.Extensions.DataIngestion.MarkdownParser.GetCells(Table table, String outputContent)
         at Microsoft.Extensions.DataIngestion.MarkdownParser.MapBlock(String documentMarkdown, Boolean previousWasBreak, Block block)
         at Microsoft.Extensions.DataIngestion.MarkdownParser.Map(MarkdownDocument markdownDocument, String documentMarkdown, String identifier)
         at Microsoft.Extensions.DataIngestion.MarkdownParser.Parse(String markdown, String identifier)
         at Microsoft.Extensions.DataIngestion.MarkdownReader.ReadAsync(Stream source, String identifier, String mediaType, CancellationToken cancellationToken)
         at Microsoft.Extensions.DataIngestion.IngestionDocumentReader.ReadAsync(FileInfo source, String identifier, String mediaType, CancellationToken cancellationToken)
         at Microsoft.Extensions.DataIngestion.IngestionPipeline`1.ProcessAsync(IEnumerable`1 files, Activity rootActivity, CancellationToken cancellationToken)+MoveNext()

Ideally we would recognize this pattern and map it to a valid table structure.
</issue_description>

<agent_instructions>I am referring to the code that is present here: https://github.com/dotnet/extensions/tree/main/src/Libraries/Microsoft.Extensions.DataIngestion.Markdig Don't forget to add a test with repro.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [MEDI] Parsing markdown fails for tables without trailing | #7132

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Microsoft Reviewers: Open in CodeFlow

[adamsitnik]

It looks great! thanks

[Copilot]

Pull request overview

This PR fixes an IndexOutOfRangeException that occurred when parsing markdown tables without trailing pipe (|) characters. The Markdig library parses these tables differently, causing table.ColumnDefinitions.Count to return inconsistent values: for tables with trailing pipes it returns actualColumns + 1, but for tables without trailing pipes it returns the actual column count. The previous code assumed all tables needed a -1 adjustment, which failed for tables without trailing pipes.

Key Changes

• Replaced hardcoded table.ColumnDefinitions.Count - 1 with a dynamic GetColumnCount() helper function that calculates columns by examining actual row cells and summing their ColumnSpan values
• Added comprehensive test case SupportsTablesWithoutTrailingPipes() demonstrating the fix works for tables with empty cells and multi-row scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Libraries/Microsoft.Extensions.DataIngestion.Markdig/MarkdownParser.cs	Introduces GetColumnCount() local function to dynamically calculate column count by iterating through table rows and summing cell column spans, replacing unreliable ColumnDefinitions.Count - 1 hardcode
test/Libraries/Microsoft.Extensions.DataIngestion.Tests/Readers/MarkdownReaderTests.cs	Adds test case with real-world table from runtime docs featuring tables without trailing pipes, empty cells, and 10 rows to validate the fix