不过使用独立的Startup类有着额外的好处，Startup类可以被包含在与Program类不用的的程序集中。然后通过WebHostOptions类中StartupAssembly属性的设定及其它相关处理，完成UseStartup方法同样的功能。
private IServiceCollection BuildCommonServices(out AggregateException hostingStartupErrors) { ... if (!string.IsNullOrEmpty(_options.StartupAssembly)) { try { var startupType = StartupLoader.FindStartupType(_options.StartupAssembly, _hostingEnvironment.EnvironmentName); if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo())) { services.AddSingleton(typeof(IStartup), startupType); } else { services.AddSingleton(typeof(IStartup), sp => { var hostingEnvironment = sp.GetRequiredService<IHostingEnvironment>(); var methods = StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName); return new ConventionBasedStartup(methods); }); } } ... } ... return services; }