OpenAPI support in ASP.NET Core API apps

ASP.NET Core supports the generation of OpenAPI documents in controller-based and minimal APIs apps. The OpenAPI specification is a programming language-agnostic standard for documenting HTTP APIs. This standard is supported in ASP.NET Core apps through a combination of built-in APIs and open-source libraries. There are three key aspects to OpenAPI integration in an application:

• Generating information about the endpoints in the app.
• Gathering the information into a format that matches the OpenAPI schema.
• Exposing the generated OpenAPI document via a visual UI or a serialized file.

ASP.NET Core apps provide built-in support for generating information about endpoints in an app via the Microsoft.AspNetCore.OpenApi package.

The following code is generated by the ASP.NET Core minimal web API template and uses OpenAPI:

using Microsoft.AspNetCore.OpenApi;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddOpenApi();

var app = builder.Build();

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

app.UseHttpsRedirection();

var summaries = new[]
{
    "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
};

app.MapGet("/weatherforecast", () =>
{
    var forecast = Enumerable.Range(1, 5).Select(index =>
        new WeatherForecast
        (
            DateTime.Now.AddDays(index),
            Random.Shared.Next(-20, 55),
            summaries[Random.Shared.Next(summaries.Length)]
        ))
        .ToArray();
    return forecast;
})
.WithName("GetWeatherForecast");

app.Run();

internal record WeatherForecast(DateTime Date, int TemperatureC, string? Summary)
{
    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);
}

In the preceding highlighted code:

• AddOpenApi registers services required for OpenAPI document generation into the application's DI container.
• MapOpenApi adds an endpoint into the application for viewing the OpenAPI document serialized into JSON. The OpenAPI endpoint is restricted to the development environment to minimize the risk of exposing sensitive information and reduce the vulnerabilities in production.

Microsoft.AspNetCore.OpenApi NuGet package

The Microsoft.AspNetCore.OpenApi package provides the following features:

• Support for generating OpenAPI documents at run time and accessing them via an endpoint on the application.
• Support for "transformer" APIs that allow modifying the generated document.

To use the Microsoft.AspNetCore.OpenApi package, add it as a PackageReference to a project file:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

  <PropertyGroup>
    <OpenApiGenerateDocuments>true</OpenApiGenerateDocuments>
    <OpenApiDocumentsDirectory>$(MSBuildProjectDirectory)</OpenApiDocumentsDirectory>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="9.0.*-*" />
    <PackageReference Include="Microsoft.Extensions.ApiDescription.Server" Version="9.0.*-*">
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
      <PrivateAssets>all</PrivateAssets>
    </PackageReference>
  </ItemGroup>

</Project>

To learn more about the Microsoft.AspNetCore.OpenApi package, see Generate OpenAPI documents.

Microsoft.Extensions.ApiDescription.Server NuGet package

The Microsoft.Extensions.ApiDescription.Server package provides support for generating OpenAPI documents at build time and serializing them.

To use Microsoft.Extensions.ApiDescription.Server, add it as a PackageReference to a project file. Document generation at build time is enabled by setting the OpenApiGenerateDocuments property. By default, the generated OpenAPI document is saved to the obj directory, but you can customize the output directory by setting the OpenApiDocumentsDirectory property.

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

  <PropertyGroup>
    <OpenApiGenerateDocuments>true</OpenApiGenerateDocuments>
    <OpenApiDocumentsDirectory>$(MSBuildProjectDirectory)</OpenApiDocumentsDirectory>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="9.0.*-*" />
    <PackageReference Include="Microsoft.Extensions.ApiDescription.Server" Version="9.0.*-*">
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
      <PrivateAssets>all</PrivateAssets>
    </PackageReference>
  </ItemGroup>

</Project>

ASP.NET Core OpenAPI source code on GitHub

• AddOpenApi
• OpenApiDocumentService
• OpenApiOptions

Additional Resources

• Authentication and authorization in minimal APIs