UnionGenerator

Compile-time discriminated unions for C# with zero runtime overhead.

Eliminate boilerplate, enable type-safe error handling, and leverage exhaustive pattern matching—all without exceptions or nullable chains.

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🚀 Quick Start

1. Installation

dotnet add package UnionGenerator

2. Define Your Union

using UnionGenerator.Attributes;

[GenerateUnion]
public partial class Result<T, E>
{
    public static Result<T, E> Ok(T value) => new OkCase(value);
    public static Result<T, E> Error(E error) => new ErrorCase(error);
}

3. Use It

var result = Result<int, string>.Ok(42);

// Pattern matching
var message = result switch
{
    { IsSuccess: true, Data: var data } => $"Success: {data}",
    { IsSuccess: false, Error: var error } => $"Error: {error}",
};

// Or use Match method
string message = result.Match(
    ok: data => $"Success: {data}",
    error: err => $"Error: {err}"
);

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✨ Features

Feature	Description
🔧 Source Generation	Zero runtime reflection, pure compile-time generated code
🎯 Type Safety	Exhaustive pattern matching with compiler-enforced case coverage
🚀 Zero Overhead	No allocations, no boxing, minimal memory footprint
🔗 ASP.NET Core Integration	Automatic ProblemDetails (RFC 7807) conversion
💾 EF Core Support	JSON value converters for database persistence
✅ FluentValidation Integration	Seamless validation error mapping
🔄 OneOf Compatibility	Drop-in replacement for OneOf library
📊 Roslyn Analyzers	Compile-time diagnostics and code fixes

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📦 Packages

Core Package

Package	Description	NuGet
UnionGenerator	Core source generator and attributes

Integration Packages

Package	Description	NuGet
UnionGenerator.AspNetCore	ASP.NET Core ProblemDetails integration
UnionGenerator.EntityFrameworkCore	EF Core value converters for unions
UnionGenerator.FluentValidation	FluentValidation error mapping
UnionGenerator.OneOfCompat	OneOf drop-in replacement

Analyzer Packages

Package	Description	NuGet
UnionGenerator.Analyzers	Roslyn analyzers for union usage
UnionGenerator.Analyzers.CodeFixes	Code fix providers

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📚 Documentation

Core Documentation

• Core Library Documentation - Complete guide to union generation, patterns, and best practices
• Examples Overview - Quick reference for all examples

Integration Guides

Integration	Documentation	Example Project
ASP.NET Core	View Docs	aspnetcore-example
Entity Framework Core	View Docs	entityframework-example
FluentValidation	View Docs	fluentvalidation-example
JSON Serialization	-	json-example
OneOf Migration	View Docs	oneof-example

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🎯 Example Projects

All examples are production-ready and demonstrate real-world patterns:

ASP.NET Core Example

Complete REST API with automatic ProblemDetails conversion, Swagger integration, and controller/minimal API examples.

cd examples/aspnetcore-example
dotnet run
# Navigate to https://localhost:5001/swagger

Entity Framework Core Example

Database persistence with JSON value converters, CRUD operations, and query patterns.

cd examples/entityframework-example
dotnet run

FluentValidation Example

Declarative validation with automatic error conversion to ProblemDetails format.

cd examples/fluentvalidation-example
dotnet run

JSON Serialization Example

System.Text.Json integration with roundtrip serialization of union types.

cd examples/json-example
dotnet run

OneOf Compatibility Example

Drop-in replacement demonstration for migrating from OneOf library.

cd examples/oneof-example
dotnet run

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🏗️ Project Structure

UnionGenerator/
├── src/
│   ├── UnionGenerator/                    # Core source generator
│   ├── UnionGenerator.Analyzers/          # Roslyn analyzers
│   ├── UnionGenerator.Analyzers.CodeFixes/# Code fix providers
│   ├── UnionGenerator.AspNetCore/         # ASP.NET Core integration
│   ├── UnionGenerator.EntityFrameworkCore/# EF Core integration
│   ├── UnionGenerator.FluentValidation/   # FluentValidation integration
│   ├── UnionGenerator.OneOfCompat/        # OneOf compatibility
│   └── UnionGenerator.OneOfExtensions/    # OneOf extensions
├── examples/                              # Production-ready examples
│   ├── aspnetcore-example/
│   ├── entityframework-example/
│   ├── fluentvalidation-example/
│   ├── json-example/
│   └── oneof-example/
├── tests/                                 # Comprehensive test suite
│   ├── UnionGenerator.Tests/
│   ├── UnionGenerator.AspNetCore.Tests/
│   ├── UnionGenerator.EntityFrameworkCore.Tests/
│   └── UnionGenerator.FluentValidation.Tests/
└── docs/                                  # Additional documentation

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💡 Common Use Cases

✅ Error Handling Without Exceptions

[GenerateUnion]
public partial class Result<T, E>
{
    public static Result<T, E> Ok(T value) => new OkCase(value);
    public static Result<T, E> Error(E error) => new ErrorCase(error);
}

public Result<User, string> GetUser(int id)
{
    if (id <= 0)
    {
        return Result<User, string>.Error("Invalid ID");
    }
    
    var user = _userService.FindById(id);
    return user != null 
        ? Result<User, string>.Ok(user)
        : Result<User, string>.Error("User not found");
}

✅ API Response Types

[GenerateUnion]
public partial class ApiResponse<T>
{
    public static ApiResponse<T> Success(T data) => new SuccessCase(data);
    public static ApiResponse<T> NotFound(string message) => new NotFoundCase(message);
    public static ApiResponse<T> Unauthorized() => new UnauthorizedCase();
    public static ApiResponse<T> ValidationError(Dictionary<string, string[]> errors) 
        => new ValidationErrorCase(errors);
}

✅ State Machines

[GenerateUnion]
public partial class OrderState
{
    public static OrderState Pending() => new PendingCase();
    public static OrderState Processing(string workerId) => new ProcessingCase(workerId);
    public static OrderState Completed(DateTime completedAt) => new CompletedCase(completedAt);
    public static OrderState Cancelled(string reason) => new CancelledCase(reason);
}

✅ Optional Values (Maybe Pattern)

[GenerateUnion]
public partial class Option<T>
{
    public static Option<T> Some(T value) => new SomeCase(value);
    public static Option<T> None() => new NoneCase();
}

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🔬 Why UnionGenerator?

The Problem

Traditional C# approaches for handling multiple return types are problematic:

Approach	Issues
Nullable types	Lose type information, awkward null-checking chains
Exceptions	Performance overhead, lose error context, not for control flow
Out parameters	Unidiomatic, not composable
Custom base classes	Boilerplate-heavy, no exhaustiveness checking

The Solution

Discriminated unions provide:

• ✅ Type Safety - Compiler-enforced exhaustive matching
• ✅ Performance - Zero runtime overhead, no exceptions
• ✅ Clarity - Intent is explicit, errors are values
• ✅ Composability - Natural functional patterns
• ✅ Maintainability - Less boilerplate, more focus on logic

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🛠️ Building the Project

Prerequisites

• .NET 8.0 SDK or later
• JetBrains Rider (recommended) or Visual Studio 2022+

Build Commands

# Clone the repository
git clone https://github.com/yourusername/UnionGenerator.git
cd UnionGenerator

# Restore dependencies
dotnet restore

# Build all projects
dotnet build

# Run tests
dotnet test

# Pack NuGet packages
dotnet pack -c Release -o nupkgs

Running Examples

Each example project can be run independently:

# ASP.NET Core example
cd examples/aspnetcore-example
dotnet run

# Entity Framework Core example
cd examples/entityframework-example
dotnet run

# All examples follow the same pattern

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🧪 Testing

# Run all tests
dotnet test

# Run specific test project
dotnet test tests/UnionGenerator.Tests/

# Run with code coverage
dotnet test --collect:"XPlat Code Coverage"

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📤 Publishing to NuGet

This project uses GitHub Actions for automated NuGet package publishing.

Quick Publish

# Create and push a version tag
git tag -a v0.2.0 -m "Release version 0.2.0"
git push origin v0.2.0

# GitHub Actions will automatically:
# ✅ Build and test
# ✅ Pack NuGet packages
# ✅ Publish to NuGet.org
# ✅ Create GitHub Release

Setup Required

1. NuGet API Key: Add NUGET_API_KEY secret to GitHub repository settings
2. Full Instructions: See Publishing Guide

Publishing Methods

• Automatic (Recommended): Push a version tag (e.g., v0.1.0)
• Manual: Use GitHub Actions workflow dispatch UI

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🤝 Contributing

Contributions are welcome! Please:

1. Fork the repository
2. Create a feature branch (git checkout -b feature/amazing-feature)
3. Commit your changes (git commit -m 'Add amazing feature')
4. Push to the branch (git push origin feature/amazing-feature)
5. Open a Pull Request

Development Guidelines

• Follow the existing code style (Rider/ReSharper conventions)
• Add tests for new features
• Update documentation as needed
• Keep commits focused and atomic

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📄 License

This project is licensed under the MIT License - see the LICENSE file for details.

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🙏 Acknowledgments

• Inspired by F#'s discriminated unions and Rust's enums
• Built with Roslyn source generators
• Community feedback and contributions

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📞 Support & Community

• Issues: GitHub Issues
• Discussions: GitHub Discussions
• Documentation: Core Docs | Examples

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Made with ❤️ for the C# community