Generate OpenApi.json on build from ASP.NET 8 Minimal API

GenerateOpenApiOnBuildFromNet8MinimalApi.md

To generate OpenApi.json on build from an ASP.NET 8 Minimal API, follow these steps:

1. In Visual Studio for Windows 17.7.0 or later, create a new ASP.NET Core API project

2. Follow these instructions to install Swashbuckle.AspNetCore.Cli as a local dotnet tool in your project

3. Add these NuGet packages (or later versions) to the project:

  <ItemGroup>
  	<PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="8.0.0-preview.4.23260.4" />
  	<PackageReference Include="Swashbuckle.AspNetCore" Version="6.5.0" />
  </ItemGroup>

4. Add this target to the .csproj file:

<!-- Generate openapi.json on build; see
     https://github.com/domaindrivendev/Swashbuckle.AspNetCore#using-the-tool-with-the-net-core-30-sdk-or-later -->
<Target Name="OpenAPI" AfterTargets="Build" Condition="$(Configuration)=='Debug'">
    <Exec Command="dotnet swagger tofile --output openapi.json $(TargetPath) v1" WorkingDirectory="$(TargetDir)" />
</Target>

5. In Program.cs, add the OpenApi services:

   builder.Services.AddEndpointsApiExplorer()
                   .AddSwaggerGen();

   and append .WithOpenApi() to the MapGroup() statement:

   var todosApi = app.MapGroup("/todos").WithOpenApi();

   (optional) if you also want to generate the Swagger UI under the /swagger url, add:

   if (app.Environment.IsDevelopment()) _ = app.UseSwagger().UseSwaggerUI();

6. Build the project; the build output will contain a line like:
   Swagger JSON/YAML successfully written to ...\bin\Debug\net8.0\openapi.json