IServiceCollection services = new ServiceCollection();
services.AddAsyncApi();
var serviceProvider = services.BuildServiceProvider();
var builder = serviceProvider.GetRequiredService<IAsyncApiDocumentBuilder>();
var document = builder
    .UseAsyncApi("2.1.0")
    .WithTitle("Streetlights API")
    .WithVersion("1.0.0")
    .WithDescription("The Smartylighting Streetlights API allows you to remotely manage the city lights.")
    .WithLicense("Apache 2.0", new Uri("https://www.apache.org/licenses/LICENSE-2.0"))
    .UseServer("mosquitto", server => server
        .WithUrl(new Uri("mqtt://test.mosquitto.org"))
        .WithProtocol("mqtt"))
    .UseChannel("light/measured", channel => channel
        .DefinePublishOperation(operation => operation
            .WithSummary("Inform about environmental lighting conditions for a particular streetlight.")
            .WithOperationId("onLightMeasured")
            .UseMessage(message => message
                .WithName("LightMeasured")
                .OfType<LightMeasuredEvent>())))
    .Build();

var writer = serviceProvider.GetRequiredService<IAsyncApiDocumentWriter>();
using MemoryStream stream = new();
await writer.WriteAsync(document, stream, AsyncApiDocumentFormat.Yaml, cancellationToken);

var reader = serviceProvider.GetRequiredService<IAsyncApiDocumentReader>();
AsyncApiDocument asyncApi = await reader.ReadAsync(stream, cancellationToken);

[AsyncApi("Streetlights API", "1.0.0", Description = "The Smartylighting Streetlights API allows you to remotely manage the city lights.", LicenseName = "Apache 2.0", LicenseUrl = "https://www.apache.org/licenses/LICENSE-2.0")]
    public class StreetLightsService
        : BackgroundService
    {

        ... //Omitted for brevity

        [Channel("light/measured"), PublishOperation(OperationId = "onLightMeasured", Summary = "Inform about environmental lighting conditions for a particular streetlight")]
        public async Task PublishLightMeasured(LightMeasuredEvent e)
        {
            MqttApplicationMessage message = new()
            {
                Topic = "onLightMeasured",
                ContentType = "application/json",
                Payload = Encoding.UTF8.GetBytes(await this.Serializer.SerializeAsync(e))
            };
            await this.MqttClient.PublishAsync(message);
        }

        [Channel("light/measured"), SubscribeOperation(OperationId = "lightMeasuredEvent", Summary = "Inform about environmental lighting conditions for a particular streetlight")]
        protected async Task OnLightMeasured(LightMeasuredEvent e)
        {
            this.Logger.LogInformation($"Event received:{Environment.NewLine}{await this.Serializer.SerializeAsync(e)}");
        }

        ...

    }

var generator = serviceProvider.GetRequiredService<IAsyncApiDocumentGenerator>();
var options = new AsyncApiDocumentGenerationOptions()
{
    DefaultConfiguration = builder =>
    {
        //Setup the document by configuring servers, for example
    }
};
IEnumerable<AsyncApiDocument> documents = generator.GenerateAsync(typeof(StreetLightsService), options);

//Startup.cs

public class Startup
{

    ...

    public void ConfigureServices(IServiceCollection services)
    {
        ...
        //Registers and configures the AsyncAPI code-first generation
        services.AddAsyncApiGeneration(builder => 
            builder.WithMarkupType<StreetLightsService>()
                .UseDefaultConfiguration(asyncApi =>
                {
                    asyncApi.UseServer("mosquitto", server => server
                        .WithUrl(new Uri("mqtt://test.mosquitto.org"))
                        .WithProtocol(AsyncApiProtocols.Mqtt));
                }));
        ...
    }

    public void Configure(IApplicationBuilder app)
    {
        ...
        //Adds the middleware used to serve AsyncAPI documents
        app.UseAsyncApiGeneration();
        ...
    }

}

services.AddAsyncApiUI();