The Future of API Documentation in .NET 9: Moving Beyond Swagger

Shift away from Swagger as the default tool for API documentation

API documentation is the heart of every backend service, and with each .NET release, there are exciting shifts in how we document, test, and understand APIs. With .NET 9 on the horizon, the way developers approach API documentation is poised to transform, offering a much richer, more intuitive experience beyond the capabilities of Swagger.

To compare the new API documentation capabilities in .NET 9 with Swagger, let’s focus on some hands-on examples. In .NET 9, changes in source generators, OpenAPI extensions, multi-protocol support, and AI-driven interactive documentation give us code-level upgrades for dynamic and context-aware API documentation. Here’s a breakdown of how these changes can replace or extend Swagger features, along with practical code snippets to illustrate what’s different in .NET 9.

1. Auto-Generated API Documentation with Source Generators

Source Generators in .NET 9 can automatically create detailed documentation based on your code, significantly reducing the need for Swagger’s manual setup.

Example: Generating Endpoint Documentation

Instead of manually annotating API endpoints, we can use source generators to automatically generate documentation at compile time.

public class ProductController : ControllerBase
{
    [HttpGet("{id}")]
    [ProducesResponseType(typeof(Product), StatusCodes.Status200OK)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public IActionResult GetProduct(int id)
    {
        var product = productService.GetProduct(id);
        return product != null ? Ok(product) : NotFound();
    }
}

With source generators, we no longer need explicit comments for each endpoint. The generator will analyze attributes, like [ProducesResponseType], to produce documentation automatically. This reduces code duplication and improves the accuracy of documentation by directly pulling from attributes.

What’s New in .NET 9:

The generator produces OpenAPI documentation by analyzing the [HttpGet], [ProducesResponseType], and other attributes, creating context-aware docs with minimal configuration, unlike Swagger which would require separate XML comments or manual input.

2. OpenAPI Extensions for Richer Documentation

.NET 9’s OpenAPI extensions allow us to create custom tags, error handling metadata, and contextual hints without needing third-party extensions or annotations like Swagger.

Example: Adding Custom Metadata

[HttpPost]
[Route("add-product")]
[OpenApiCustomTag("E-commerce API")]
[OpenApiResponseExample(typeof(ProductResponse), typeof(ProductResponseExampleProvider))]
public async Task<IActionResult> AddProduct(ProductRequest request)
{
    // Endpoint logic
}

In this example, we use custom OpenApiCustomTag and OpenApiResponseExample attributes introduced in .NET 9, where Swagger would typically require us to define these in a swagger.json file or use separate annotations.

3. Multi-Protocol API Support with gRPC and REST Documentation

.NET 9’s documentation natively supports multiple protocols (REST, gRPC, GraphQL) in a single documentation platform, moving beyond Swagger’s HTTP-focused approach.

Example: Documenting gRPC and REST Endpoints Together

With .NET 9, you can document both REST and gRPC in the same place, without any additional third-party tooling:

[HttpGet("products")]
[GrpcRoute("grpc/ProductService/GetProducts")]
[OpenApiOperation("List all products", Description = "Gets a list of all available products.")]
public async Task<IActionResult> GetProducts()
{
    // Retrieve and return products
}

By using [GrpcRoute], .NET 9 automatically integrates gRPC alongside HTTP, presenting both as part of a unified documentation, whereas Swagger typically doesn’t support gRPC without extra configuration or external tools.

4. AI-Driven Documentation Assistants

With .NET 9, you can integrate AI assistants directly into your documentation to provide real-time suggestions and example code generation. Although AI-driven features are still evolving, .NET 9 allows developers to configure these to offer suggestions or code snippets based on API usage patterns.

Example: Setting Up AI for Interactive Documentation

With .NET 9’s AI-powered assistant, you can embed real-time code suggestions or examples:

{
    "assistantConfig": {
        "enableAI": true,
        "suggestions": [
            {
                "context": "GetProduct",
                "example": "GET /api/products/{id} - Retrieves product by ID"
            }
        ]
    }
}

5. Real-Time Metrics and Error Reporting

Unlike Swagger’s static documentation, .NET 9 can integrate real-time performance and error metrics into your documentation.

Example: Real-Time API Metrics

{
    "metrics": {
        "endpoint": "/api/products",
        "averageResponseTime": "120ms",
        "errorRate": "2%",
        "lastError": "Product not found"
    }
}

This real-time metrics configuration gives developers immediate insight into endpoint performance and error trends, helping them diagnose issues faster. Swagger requires external integrations or tools for similar real-time information.

Wrapping Up

.NET 9’s new features simplify documentation while making it more dynamic, context-aware, and protocol-flexible. Moving beyond Swagger, .NET 9’s source generators, OpenAPI enhancements, multi-protocol support, and real-time metrics are setting new standards for what API documentation can achieve.