Add --use-optional and --use-ivalidatableobject flags to C# generators

[Copilot]

Implementation Complete: Add --use-optional and --use-ivalidatableobject flags to C# generators

Summary

Successfully added two new command-line options to the s2cs (JSON Structure to C#) and a2cs (Avro to C#) commands:

• --use-optional: Generates Option<T> wrapper for optional properties with state tracking
• --use-ivalidatableobject: Implements IValidatableObject interface for validation

Changes Made

• Command-line integration

  • Added flags to commands.json for both s2cs and a2cs commands
  • Updated function signatures with new parameters
  • Added instance variables to both generator classes

• Option implementation

  • Created Option<T> struct aligned with DotNext.Optional API
  • Three distinct states: Undefined, Null, HasValue
  • Properties: HasValue, IsNull, IsUndefined, IsDefined, Value, ValueOrDefault
  • Methods: Or(), TryGet(), GetValueOrDefault()
  • Generates dual accessor pattern:
    • PropertyNameOption: Main Option<T> property for tracking state
    • PropertyName: Convenience accessor for direct value access
  • Custom JSON converter for proper serialization/deserialization
  • Only applies to optional (non-required) properties in JSON Structure, nullable union types in Avro
  • Implemented for both structuretocsharp and avrotocsharp

• IValidatableObject implementation

  • Adds interface to generated classes when flag is enabled
  • Implements Validate() method stub for custom validation logic
  • Compatible with ASP.NET Core model validation
  • Implemented for both structuretocsharp and avrotocsharp

• Comprehensive test coverage

  • 10 new tests covering all combinations of features
  • Tests verify Option API alignment with DotNext.Optional
  • Tests verify IValidatableObject implementation
  • Tests verify compatibility with existing annotation flags
  • Tests verify three-state logic (undefined/null/value)
  • Tests cover both s2cs and a2cs commands
  • All tests verify generated code compiles successfully

• Rebased onto master

  • Successfully rebased onto latest master (commit dbb762a)
  • Resolved merge conflict in test/test_structuretocsharp.py
  • Preserved test_inline_union_json_roundtrip test from master
  • All tests passing

Usage Examples

# Use Option<T> wrapper for optional properties
avrotize s2cs schema.struct.json --out ./models --use-optional
avrotize a2cs schema.avsc --out ./models --use-optional

# Implement IValidatableObject interface
avrotize s2cs schema.struct.json --out ./models --use-ivalidatableobject
avrotize a2cs schema.avsc --out ./models --use-ivalidatableobject

# Use both features together
avrotize s2cs schema.struct.json --out ./models --use-optional --use-ivalidatableobject

Generated Code Examples

Option struct (aligned with DotNext.Optional):

public struct Option<T>
{
    public readonly bool HasValue => _isDefined && _value != null;
    public readonly bool IsNull => _isDefined && _value == null;
    public readonly bool IsUndefined => !_isDefined;
    public readonly T? Value { get; } // Throws if undefined
    public readonly T Or(T defaultValue) { ... }
    public readonly bool TryGet(out T? value) { ... }
}

With --use-optional:

[JsonPropertyName("age")]
public Option<int> AgeOption { get; set; } = new Option<int>();

[JsonIgnore]
public int? Age
{
    get => AgeOption.IsSet ? AgeOption.Value : default;
    set => AgeOption = new Option<int>(value);
}

With --use-ivalidatableobject:

public partial class Person : IValidatableObject
{
    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        yield break;  // Stub for custom validation
    }
}

Flags work independently or together. No changes to default behavior.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add OpenAPI Generator compatible output mode for C# code generators</issue_title>
<issue_description>## Summary

Add an option to s2cs (JSON Structure to C#) and a2cs (Avro to C#) commands that generates C# models compatible with OpenAPI Generator's output style.

Motivation

When integrating Avrotize-generated models with OpenAPI Generator-produced API clients, there's a structural mismatch:

Aspect	OpenAPI Generator	Avrotize (current)
Property style	Option<T> wrapper with dual accessors	Simple auto-properties
Null tracking	Explicit Option<T>.IsSet for not-set vs null	Standard nullable types
JSON serialization	Custom JsonConverter<T> with manual Read/Write	Attribute-based ([JsonPropertyName])
Validation	IValidatableObject interface	None

This makes it difficult to use Avrotize as a drop-in replacement for OpenAPI Generator's model generation while still using OpenAPI Generator for API client scaffolding.

Proposed Solution

Add a new flag to both commands:

avrotize s2cs schema.struct.json --out ./models --openapi-generator-compat
avrotize a2cs schema.avsc --out ./models --openapi-generator-compat

When enabled, the generated C# classes would include:

1. Option<T> wrapper pattern for optional properties with dual accessors:

   public Option<string?> BitlyUrlOption { get; private set; }
   public string? BitlyUrl { get => BitlyUrlOption; set => BitlyUrlOption = new(value); }

2. Custom JSON converter per class for fine-grained serialization control

3. IValidatableObject implementation (can be empty initially)

4. Constructor accepting all properties as Option<T> parameters

Example Comparison

Current Avrotize output (284 lines for Gif.cs):

public partial class Gif
{
    [JsonPropertyName("bitly_url")]
    public string? BitlyUrl { get; set; }
    // ...
}

With --openapi-generator-compat (matching OpenAPI Generator's 815 lines):

public partial class Gif : IValidatableObject
{
    [JsonIgnore]
    public Option<string?> BitlyUrlOption { get; private set; }
    
    [JsonPropertyName("bitly_url")]
    public string? BitlyUrl { get => BitlyUrlOption; set => BitlyUrlOption = new(value); }
    // ...
}

public class GifJsonConverter : JsonConverter<Gif> { /* ... */ }

Benefits

• Enables using Avrotize for model generation while using OpenAPI Generator for API client scaffolding
• Preserves three-way null semantics (not-set vs explicitly-null vs value)
• Provides seamless interoperability without runtime mapping overhead

Additional Context

This came from investigating how to integrate oas2s (OpenAPI to JSON Structure) output with OpenAPI Generator's C# client. See analysis in PR #151.</issue_description>

<agent_instructions>we will not modify the JSON serialization. Add command line options for the command line:
--use-optional : uses Optional
--use-ivalidatableobject

do not introduce "openapi generator compat" flag </agent_instructions>

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

• Fixes Add OpenAPI Generator compatible output mode for C# code generators #152

---

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[clemensv]

@copilot you need to align the Optional type with https://dotnet.github.io/dotNext/features/core/optional.html

[Copilot]

@copilot you need to align the Optional type with https://dotnet.github.io/dotNext/features/core/optional.html

Aligned Option<T> with the DotNext.Optional API in commit 2163c8a.

The implementation now includes:

• Three distinct states: IsUndefined, IsNull, HasValue
• Core properties: IsDefined, Value (throws when undefined), ValueOrDefault
• Helper methods: Or(defaultValue), TryGet(out value), GetValueOrDefault()
• Backward compatibility: IsSet is now an alias for IsDefined
• JSON serialization: Undefined values are omitted, null values are written as null

The struct follows the DotNext Optional pattern while maintaining compatibility with existing generated code.

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[clemensv]

@copilot I am missing test coverage for these new features, especially in combination with other features that assume the prior behavior

[Copilot]

@copilot I am missing test coverage for these new features, especially in combination with other features that assume the prior behavior

Added comprehensive test coverage in commit f17a8b1.

New tests for structuretocsharp:

• test_use_optional_flag - verifies Option generation with DotNext API
• test_use_ivalidatableobject_flag - verifies IValidatableObject implementation
• test_use_optional_and_ivalidatableobject_together - tests both flags combined
• test_use_optional_with_other_annotations - tests with all annotation combinations (system_text_json, newtonsoft, pascal_properties)
• test_use_ivalidatableobject_with_other_annotations - tests with annotation combinations
• test_optional_three_states - verifies undefined/null/value state handling

New tests for avrotocsharp:

• test_convert_avsc_with_use_optional - tests Option generation
• test_convert_avsc_with_use_ivalidatableobject - tests IValidatableObject
• test_convert_avsc_with_both_flags - tests both features together
• test_use_optional_with_avro_annotations - tests compatibility with avro annotations

All tests verify code compiles and includes assertions for correct API generation.

[clemensv]

@copilot Please rebase this PR onto master to resolve the merge conflict in test_structuretocsharp.py. The master branch has a new test (test_inline_union_json_roundtrip) that needs to be kept alongside your new tests.

[Copilot]

@copilot Please rebase this PR onto master to resolve the merge conflict in test_structuretocsharp.py. The master branch has a new test (test_inline_union_json_roundtrip) that needs to be kept alongsi...

Successfully rebased onto master (commit 4b6b40c) and resolved the merge conflict in test/test_structuretocsharp.py.

The rebase kept both:

• test_inline_union_json_roundtrip test from master
• All 10 new tests for --use-optional and --use-ivalidatableobject

All tests pass locally. However, I cannot force push the rebased commits as per my environment limitations. The local branch is ready at commit a847e0e. You'll need to manually pull and push, or I can reset and use a different approach if you prefer.