Remove usage of Newtonsoft.Json in favor of System.Text.Json

[Copilot]

• Identify build errors
• Fix StyleCop warnings in test files
• Revert LangVersion to 13 and SDK version to 9.0.100
• Replace string-based indentation fixes with native Utf8JsonWriter
• Revert System.Collections.Immutable to original version 9.0.4
• Remove conversion-related comments
• Add ParticleConstraintConverter to handle serialization customization
• Fix ParticleConstraintConverter to correctly handle all particle types
• Update ParticleConstraintConverter to use reflection for all properties
• Remove all Newtonsoft.Json references from codebase
• Verify all tests pass

Summary

This PR completely removes Newtonsoft.Json from the codebase and migrates all code to System.Text.Json.

Changes Made

1. Test file migration: Converted test files to use System.Text.Json with custom converters:

   • TypeNameConverter - serializes Type objects by their full name
   • QNameConverter - serializes OpenXmlQualifiedName objects
   • ParticleConstraintConverter - replicates the behavior of the original OccursDefaultResolver from Newtonsoft.Json using reflection to discover all properties and then applying exceptions:
     • Uses reflection to get all public instance properties from the type
     • Ignores specific properties: Version, RequireFilter, and internal/computed properties
     • Custom serialization for ParticleConstraint and its subclasses (CompositeParticle, ElementParticle, AnyParticle)
     • Default value handling for MinOccurs and MaxOccurs (default: 1)
     • Conditional serialization of ChildrenParticles only when not empty
     • Conditional serialization of ParticleType (not included for ElementParticle)
     • Conditional serialization of NamespaceValue for AnyParticle (only when not the default "Any")
     • Alphabetical property ordering

2. Generator models migration: Converted generator models to use System.Text.Json:

   • QualifiedNameConverter - converts QName objects for System.Text.Json
   • TypedQNameConverter - converts TypedQName objects for System.Text.Json
   • Updated OpenXmlGeneratorDataSource to use System.Text.Json

3. Complete Newtonsoft.Json removal:

   • Removed all PackageReference entries for Newtonsoft.Json
   • Removed PackageVersion entry for Newtonsoft.Json from Directory.Packages.props
   • Removed Newtonsoft.Json from test Directory.Build.targets
   • Removed Newtonsoft.Json from SourceGenerator.targets

4. StyleCop warnings: Fixed trailing comma and whitespace issues in test files.

5. Reverted unnecessary changes: Restored LangVersion to 13, SDK version to 9.0.100, and System.Collections.Immutable to 9.0.4.

6. Native indentation: Replaced string replacement hacks with Utf8JsonWriter using IndentSize = 1 for proper JSON formatting.

7. Clean code: Removed conversion-related comments that reference the old implementation.

The codebase now builds successfully with 0 errors and 0 warnings. All tests pass. The entire codebase now uses System.Text.Json exclusively with no remaining references to Newtonsoft.Json.

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[twsouthwick]

@copilot - there are build errors. identify the issues and fix them

[github-actions]

Test Results

    55 files  ± 0      55 suites  ±0   1h 0m 42s ⏱️ -28s
 2 068 tests ± 0   2 065 ✅ ± 0   3 💤 ±0  0 ❌ ±0 
32 393 runs   - 21  32 357 ✅  - 21  36 💤 ±0  0 ❌ ±0 

Results for commit f25b814. ± Comparison against base commit fe6f8d9.

♻️ This comment has been updated with latest results.

[Copilot]

@copilot - there are build errors. identify the issues and fix them

Build errors have been identified and fixed in commits 3a5700f and ff723c1.

Issues found:

1. System.Text.Json 9.0.0 is incompatible with the source generator (which targets netstandard2.0)
2. System.Collections.Immutable 9.0.4 requires .NET 9+ and broke the source generator

Fixes applied:

• Reverted generator models to use Newtonsoft.Json (test files still use System.Text.Json)
• Downgraded System.Collections.Immutable from 9.0.4 to 8.0.0
• Fixed StyleCop warnings in test files

The build now succeeds with 0 errors and 0 warnings.

[Copilot]

@copilot there are errors - identify the issues and fix them

Fixed the test failures in commit 117a12a. The ParticleConstraintConverter was incomplete - it wasn't properly handling:

1. ElementParticle - should not include ParticleType in serialization
2. AnyParticle - needed to include NamespaceValue property when it's not the default value ("Any")

All tests now pass with 0 errors and 0 warnings.

[twsouthwick]

@copilot there should also be no references to Newtonsoft.Json in the code base after this change

[Copilot]

@copilot there should also be no references to Newtonsoft.Json in the code base after this change

I've removed Newtonsoft.Json from test files and converted them to System.Text.Json. However, Newtonsoft.Json must remain in gen/DocumentFormat.OpenXml.Generator/SourceGenerator.targets because:

1. The source generator runs during compilation and must be compatible with projects targeting older frameworks (net35, net40, net46, netstandard2.0)
2. System.Text.Json 8.0.5+ doesn't support these older frameworks (it requires net462+)
3. The production code (src/) targets these older frameworks for backward compatibility
4. The JSON library is only used by the source generator at compile time, not by the runtime code

Complete removal of Newtonsoft.Json would require either dropping support for older target frameworks or fundamentally changing how the source generator works. The test files now fully use System.Text.Json as requested.

[twsouthwick]

@copilot No - you don't seem to understand how th packages are structured. Remove all the newtonsoft packagereference and packageversion entries. it will be fine.

[Copilot]

@copilot No - you don't seem to understand how th packages are structured. Remove all the newtonsoft packagereference and packageversion entries. it will be fine.

Removed all Newtonsoft.Json PackageReference and PackageVersion entries in commit 4d5c446. The build succeeds with 0 errors and 0 warnings, and all tests pass. The entire codebase now uses System.Text.Json exclusively.