OpenApi

Built-in support for OpenAPI document generation

The OpenAPI specification is a standard for describing HTTP APIs. The standard allows developers to define the shape of APIs that can be plugged into client generators, server generators, testing tools, documentation, and more. In .NET 9, ASP.NET Core provides built-in support for generating OpenAPI documents representing controller-based or Minimal APIs via the Microsoft.AspNetCore.OpenApi package.

The following highlighted code calls:

• AddOpenApi to register the required dependencies into the app's DI container.
• MapOpenApi to register the required OpenAPI endpoints in the app's routes.

:::code language="csharp" source="~/release-notes/aspnetcore-9/samples/OpenApiExample/Program.cs" highlight="3,7":::

Install the Microsoft.AspNetCore.OpenApi package in the project using the following command:

dotnet add package Microsoft.AspNetCore.OpenApi

Run the app and navigate to openapi/v1.json to view the generated OpenAPI document:

OpenAPI documents can also be generated at build-time by adding the Microsoft.Extensions.ApiDescription.Server package:

dotnet add package Microsoft.Extensions.ApiDescription.Server

To modify the location of the emitted OpenAPI documents, set the target path in the OpenApiDocumentsDirectory property in the app's project file:

:::code language="xml" source="~/release-notes/aspnetcore-9/samples/OpenApiExample/OpenApiExample.csproj" range="9-11":::

Run dotnet build and inspect the generated JSON file in the project directory.

ASP.NET Core's built-in OpenAPI document generation provides support for various customizations and options. It provides document, operation, and schema transformers and has the ability to manage multiple OpenAPI documents for the same application.

To learn more about ASP.NET Core's new OpenAPI document capabilities, see the new Microsoft.AspNetCore.OpenApi docs.