✅ Not able to locate web api controllers in a class library

namespace Core
{
    public class Startup
    {
        private readonly IConfiguration _config;

        public Startup(IConfiguration config)
        {
            _config = config;
        }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddApiServices(_config);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app)
        {
            app.ConfigureApi(_config);
        }
    }
}

namespace Core
{
    public class Startup
    {
        private readonly IConfiguration _config;

        public Startup(IConfiguration config)
        {
            _config = config;
        }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddApiServices(_config);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app)
        {
            app.ConfigureApi(_config);
        }
    }
}

namespace Api.Extensions
{
    public static class ApiStartupExt
    {
        public static void AddApiServices(this IServiceCollection builder, IConfiguration config)
        {
        builder.AddControllers().AddApplicationPart(typeof(ApiStartupExt).Assembly);
            builder.AddSwaggerGen(c => c.SwaggerDoc("v0", new OpenApiInfo
            {
                Title = "test_service",
                Version = "v0"
            }));
        }
        public static void ConfigureApi(this IApplicationBuilder app, IConfiguration config)
        {
            if (!config["Environment"]!.Equals("Production"))
            {
                app.UseSwagger();
                app.UseSwaggerUI(c =>
                {
                    c.SwaggerEndpoint("/swagger/v1/swagger.json", "test_service v0");
                    c.RoutePrefix = string.Empty;
                });
            }

            // Map rest services to port 43000
            // In my actual application I also have GRPC endpoints on the same host
            // so I need to use the UseWhen method to avoid some issues there
            app.UseWhen(context => context.Connection.LocalPort == config.GetValue<int>("Rest:Port"),
              builder =>
              {
                  builder.UsePathBase(new PathString("/api"));
                  builder.UseRouting();
                  builder.UseEndpoints(endpoints => endpoints.MapControllers());
              }
            );
        }
    }
}

namespace Api.Extensions
{
    public static class ApiStartupExt
    {
        public static void AddApiServices(this IServiceCollection builder, IConfiguration config)
        {
        builder.AddControllers().AddApplicationPart(typeof(ApiStartupExt).Assembly);
            builder.AddSwaggerGen(c => c.SwaggerDoc("v0", new OpenApiInfo
            {
                Title = "test_service",
                Version = "v0"
            }));
        }
        public static void ConfigureApi(this IApplicationBuilder app, IConfiguration config)
        {
            if (!config["Environment"]!.Equals("Production"))
            {
                app.UseSwagger();
                app.UseSwaggerUI(c =>
                {
                    c.SwaggerEndpoint("/swagger/v1/swagger.json", "test_service v0");
                    c.RoutePrefix = string.Empty;
                });
            }

            // Map rest services to port 43000
            // In my actual application I also have GRPC endpoints on the same host
            // so I need to use the UseWhen method to avoid some issues there
            app.UseWhen(context => context.Connection.LocalPort == config.GetValue<int>("Rest:Port"),
              builder =>
              {
                  builder.UsePathBase(new PathString("/api"));
                  builder.UseRouting();
                  builder.UseEndpoints(endpoints => endpoints.MapControllers());
              }
            );
        }
    }
}

info: Microsoft.AspNetCore.Hosting.Diagnostics[1]
      Request starting HTTP/1.1 POST http://localhost:43000/api/WeatherForecast - 0
dbug: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
      Wildcard detected, all requests with hosts will be allowed.
dbug: Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware[1]
      POST requests are not supported
dbug: Microsoft.AspNetCore.Server.Kestrel.Connections[9]
      Connection id "0HMNNSN1PNNGU" completed keep alive response.
info: Microsoft.AspNetCore.Hosting.Diagnostics[2]
      Request finished HTTP/1.1 POST http://localhost:43000/api/WeatherForecast - 0 - 404 0 - 11.6784ms

info: Microsoft.AspNetCore.Hosting.Diagnostics[1]
      Request starting HTTP/1.1 POST http://localhost:43000/api/WeatherForecast - 0
dbug: Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware[0]
      Wildcard detected, all requests with hosts will be allowed.
dbug: Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware[1]
      POST requests are not supported
dbug: Microsoft.AspNetCore.Server.Kestrel.Connections[9]
      Connection id "0HMNNSN1PNNGU" completed keep alive response.
info: Microsoft.AspNetCore.Hosting.Diagnostics[2]
      Request finished HTTP/1.1 POST http://localhost:43000/api/WeatherForecast - 0 - 404 0 - 11.6784ms

{
  "My": {
    "Foo": {
      "Kix": 5
    }
  }
}

{
  "My": {
    "Foo": {
      "Kix": 5
    }
  }
}

public class FooOptions
{
  public const string SectionName = "My:Foo";

  public string Bar {get;set;} = "default value for bar";
  public int Kix {get;set;} = -1;
  public DateTime? Pouet {get;set;} = default;
}

public class FooOptions
{
  public const string SectionName = "My:Foo";

  public string Bar {get;set;} = "default value for bar";
  public int Kix {get;set;} = -1;
  public DateTime? Pouet {get;set;} = default;
}

namespace Microsoft.Extensions.DependencyInjection; // <==== recommanded for service.Add so that you don't clutter Startup file

public class FooServiceCollectionExtensions
{
  public static IServiceCollection AddFoo(this IServiceCollection services) =>
    services
      .AddOptions<FooOptions>()
      .BindConfiguration(FooOptions.SectionName)
      .Validate(options => options.Kix >= 0, $"The configuration key '{FooOptions.SectionName}:{nameof(Kix)}' cannot be negative")
      ;

  public static IServiceCollection AddFoo(this IServiceCollection services, Action<FooOptions> configure) =>
    services
      .AddFoo()
      .Configure(configure);

namespace Microsoft.Extensions.DependencyInjection; // <==== recommanded for service.Add so that you don't clutter Startup file

public class FooServiceCollectionExtensions
{
  public static IServiceCollection AddFoo(this IServiceCollection services) =>
    services
      .AddOptions<FooOptions>()
      .BindConfiguration(FooOptions.SectionName)
      .Validate(options => options.Kix >= 0, $"The configuration key '{FooOptions.SectionName}:{nameof(Kix)}' cannot be negative")
      ;

  public static IServiceCollection AddFoo(this IServiceCollection services, Action<FooOptions> configure) =>
    services
      .AddFoo()
      .Configure(configure);

services.AddFoo();
// or
services.AddFoo(fooOptions => fooOptions.Kix = 12);

services.AddFoo();
// or
services.AddFoo(fooOptions => fooOptions.Kix = 12);

public class Bar
{
  private readonly FooOptions _fooOptions;

  // .Value in ctor is fine only if it's always ever a non-changing value (no reload and/or no scoped resolution)
  public Bar(IOptions<FooOptions> fooOptions)
    => _fooOptions = fooOptions.Value;
}

public class Bar
{
  private readonly FooOptions _fooOptions;

  // .Value in ctor is fine only if it's always ever a non-changing value (no reload and/or no scoped resolution)
  public Bar(IOptions<FooOptions> fooOptions)
    => _fooOptions = fooOptions.Value;
}