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OpenAPI Code Generator

Introduction

OpenAPI Code Generator is a .NET tool that transforms OpenAPI 3.x specifications into clean, modern C# source code. It generates record types, enums, and primitive type aliases with built-in System.Text.Json attributes and generated converters where needed.

The Problem

Section titled “The Problem”

When working with REST APIs described by OpenAPI specifications, you need C# types that match the API’s data contracts. Manually creating and maintaining these models is tedious, error-prone, and a waste of developer time — especially for large APIs with hundreds of schemas.

The Solution

Section titled “The Solution”

OpenAPI Code Generator reads your OpenAPI spec and produces idiomatic C# code automatically:

  • Records with init-only properties for API models
  • Enums with string-backed JSON serialization
  • Type aliases for simple wrapper types
  • Nullable reference types for optional fields
  • XML doc comments from OpenAPI descriptions

Key Features

Section titled “Key Features”

Modern C# Patterns

Section titled “Modern C# Patterns”

Generated code uses record types, required properties, init accessors, and nullable reference types — following current C# best practices.

Zero Runtime Dependencies

Section titled “Zero Runtime Dependencies”

The generated code only depends on System.Text.Json, which is built into .NET. No additional NuGet packages are needed in your consuming project.

Primitive Alias Flexibility

Section titled “Primitive Alias Flexibility”

Primitive component aliases are emitted as strongly typed wrapper structs by default and include generated System.Text.Json converters. If you prefer plain primitives at usage sites, enable InlinePrimitiveTypeAliases or pass --inline-type-aliases in the CLI.

Flexible Usage

Section titled “Flexible Usage”

Use it as a CLI tool for quick one-off generation, or reference the core library for integration into your own build pipeline or tooling.

How It Works

Section titled “How It Works”
┌─────────────────┐     ┌──────────────────┐     ┌─────────────────┐
│  OpenAPI Spec   │────▶│  Code Generator  │────▶│   C# Source     │
│  (JSON / YAML)  │     │                  │     │   (.cs file)    │
└─────────────────┘     └──────────────────┘     └─────────────────┘
  1. Parse — The OpenAPI specification is parsed using the official Microsoft OpenAPI library
  2. Resolve — Schema types are resolved to C# types, handling $ref, composition (allOf/oneOf/anyOf), and primitives
  3. Emit — Clean C# source code is generated with proper naming, formatting, and serialization attributes

Next Steps

Section titled “Next Steps”