OpenAPI Documentation in .NET 9

Introduction to OpenAPI in ASP.NET Core 9

With the release of .NET 9, the ASP.NET Core team has decided to remove built-in Swagger support (Swashbuckle) for several key reasons:

• Maintenance Challenges: Swashbuckle is no longer actively maintained, lacks updates, and doesn’t have an official release for .NET 8.
• Native Metadata Support: ASP.NET Core now includes built-in metadata to describe APIs, reducing the need for external tools.
• Focus on OpenAPI: Microsoft is enhancing OpenAPI support natively with Microsoft.AspNetCore.OpenApi to provide seamless documentation generation.
• Modern Alternatives: Tools like .http files and the Endpoints Explorer in Visual Studio allow testing and exploration without relying on third-party packages.
• Encouraging Innovation: Removing Swashbuckle as a default encourages community-driven tools that better suit developer needs.

This project demonstrates how to adapt to these changes by leveraging modern alternatives and provides practical examples to help you get started.

What’s Included in This Project

To help developers transition to the new direction of ASP.NET Core 9, this repository includes three samples:

1. Using .http Files: A lightweight and modern way to test APIs in Visual Studio and VS Code.
2. Re-adding Swagger Support: A sample for those who still want to use Swagger (Swashbuckle) for API documentation.
3. Introducing Scalar: A powerful alternative to Swagger with additional features, a modern UI, and rich API exploration capabilities.

Using .http Files

.http files allow you to define and test HTTP requests directly from your editor, such as Visual Studio Code with the REST Client extension or Visual Studio.

Why .http Files?

• Lightweight, simple, and human-readable.
• Allows quick testing of endpoints.
• Supports variables and response reuse in Visual Studio Code.

Example: A Simple .http File

@hostaddress = https://localhost:5555/calculation
@value1 = 20
@value2 = 22

### Add Request
# @name add
POST {{hostaddress}}/add
Content-Type: application/json

{
  "value1": {{value1}},
  "value2": {{value2}}
}

### Reuse Response
@addresult = {{add.response.body.result}}

POST {{hostaddress}}/add
Content-Type: application/json

{
  "value1": {{addresult}},
  "value2": {{addresult}}
}

Notes:

• In Visual Studio Code, the REST Client extension supports variables and response reuse (e.g., add.response.body.result).
• In Visual Studio, response variables are not yet supported, but you can still execute requests and debug effectively.

Example UI in Visual Studio:

Re-Adding Swagger Support

While Swagger has been removed from .NET 9, you can easily add it back using Swashbuckle.

Steps to Add Swagger:

1. Install Required NuGet Packages:

   dotnet add package Microsoft.AspNetCore.OpenApi
   dotnet add package Swashbuckle.AspNetCore
   

2. Update Program.cs:

   builder.Services.AddSwaggerGen(options =>
   {
       options.SwaggerDoc("v1", new OpenApiInfo
       {
           Title = "Sample API",
           Version = "v1",
           Description = "API to demonstrate Swagger integration."
       });
   });
   
   // some code
   
   if (app.Environment.IsDevelopment())
   {
       app.UseSwagger();
       app.UseSwaggerUI();
   }
   

Swagger UI Preview:

Swagger provides a clear interface to explore and test your API endpoints.

Introducing Scalar: A Modern Alternative

Scalar is an open-source API platform that takes API documentation and testing to the next level. It offers modern features, an intuitive user experience, and a sleek interface (with dark mode for real engineers!).

Why Use Scalar?

• Modern REST Client: Test and interact with APIs seamlessly.
• Beautiful API References: Generates clean, readable API documentation.
• Code Generation: Generate samples in 25+ languages or frameworks.

Scalar Example Output

C# Example:

using System.Net.Http.Headers;

var client = new HttpClient();
var request = new HttpRequestMessage
{
    Method = HttpMethod.Get,
    RequestUri = new Uri("https://localhost:5555/api/v1/time"),
};
using (var response = await client.SendAsync(request))
{
    response.EnsureSuccessStatusCode();
    var body = await response.Content.ReadAsStringAsync();
    Console.WriteLine(body);
}

JavaScript/jQuery Example:

const settings = {
  async: true,
  crossDomain: true,
  url: 'https://localhost:5555/api/v1/time',
  method: 'GET',
  headers: {}
};

$.ajax(settings).done(function (response) {
  console.log(response);
});

Adding Scalar to Your Project

1. Install Scalar NuGet Package:

   dotnet add package Scalar.AspNetCore
   

2. Update Program.cs:

   if (app.Environment.IsDevelopment())
   {
       app.MapScalarApiReference();
   }
   

Scalar UI Preview

Summary

With .NET 9, Microsoft has shifted focus to native OpenAPI support, removing the dependency on Swashbuckle. However, with .http files, Swagger, and Scalar, you have powerful tools at your disposal:

1. Use .http files for lightweight API testing.
2. Re-integrate Swagger for familiar interactive documentation.
3. Explore Scalar for a modern, feature-rich alternative with advanced capabilities.

Source Code

Find all samples in the GitHub repository: OpenAPI Documentation in .NET 9.