.NET核心中的依赖项解析

我创建了一个示例来测试我对.NET Core上依赖项解析(特别是它们的顺序,如果有的话)的理解。
它确实澄清了一些问题,但给我留下了一个没有答案的问题。

请考虑以下辅助服务模板代码:

Foo公司

        public class Foo : IFoo
        {
            private readonly IGoo Goo;
    
            public Foo(IGoo goo)
            {
                Goo = goo;
                Console.WriteLine($"Instance of [{this.GetType().Name}] being created...");
            }
        }
    
        public interface IFoo
        {
        }

古

        public class Goo : IGoo
        {
            private readonly IWoo Woo;
            public Goo(IWoo woo)
            {
                Woo = woo;
                Console.WriteLine($"Instance of [{this.GetType().Name}] being created...");
            }
        }
    
        public interface IGoo
        {
        }

求爱

        public class Woo : IWoo
        {
            IFoo Foo;
            public Woo(IFoo foo)
            {
                Foo = foo;
                Console.WriteLine($"Instance of [{this.GetType().Name}] being created...");
            }
        }
    
        public interface IWoo
        {
        }

工人服务

        public class Worker : BackgroundService
        {
            private readonly ILogger<Worker> _logger;
            //private readonly IFoo Foo;
            private readonly IGoo Goo;
            //private readonly IWoo Woo;
    
            public Worker(ILogger<Worker> logger, /*IFoo foo,*/ IGoo goo/*, IWoo woo*/)
            {
                _logger = logger;
                //Foo = foo;
                Goo = goo;
                //Woo = woo;
            }
    
            protected override async Task ExecuteAsync(CancellationToken stoppingToken)
            {
                while (!stoppingToken.IsCancellationRequested)
                {
                    _logger.LogInformation("Worker running at: {time}", DateTimeOffset.Now);
                    await Task.Delay(1000, stoppingToken);
                }
            }
        }

程序.cs

        public class Program
        {
            public static void Main(string[] args)
            {
                CreateHostBuilder(args).Build().Run();
            }
    
            public static IHostBuilder CreateHostBuilder(string[] args) =>
                Host.CreateDefaultBuilder(args)
                    .ConfigureServices((hostContext, services) =>
                    {
                        services.AddHostedService<Worker>();
                        services.AddSingleton<IFoo, Foo>();
                        services.AddSingleton<IGoo, Goo>();
                        services.AddSingleton<IWoo, Woo>();
                    });
        }

起初,我想确保内部的依赖顺序声明 ConfigureServices 无关紧要。
然而,运行上面的代码会打印 Instance of [Goo] being created... 只有
我希望 Instance of [Woo] being created... 也打印,因为它是的依赖项 Goo ,已解决。
我希望 Woo 也是必需的解析,因此调用它的构造函数。但这种情况只发生在 古 ,不适用 求爱 。

为什么?