Question

我在理解代码中的错误源时遇到了问题。我尝试着讲有关.net核心中的微服务的课程。运行构建解决方案后，我得到：

------- Project finished: CrossX.Services.Identity. Succeeded: True. Errors: 0. Warnings: 0

但是当我运行它时，我得到：

/opt/dotnet/dotnet /RiderProjects/crossx/src/CrossX.Services.Identity/bin/Debug/netcoreapp2.2/CrossX.Services.Identity.dll

Unhandled Exception: System.InvalidOperationException: Cannot resolve scoped service 'CrossX.NETCore.Commands.ICommandHandler`1[CrossX.NETCore.Commands.CreateUser]' from root provider.
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteValidator.ValidateResolution(Type serviceType, IServiceScope scope, IServiceScope rootScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at CrossX.NETCore.Services.ServiceHost.BusBuilder.SubscribeToCommand[TCommand]() in /RiderProjects/crossx/src/CrossX.NETCore/Services/ServiceHost.cs:line 78
   at CrossX.Services.Identity.Program.Main(String[] args) in /RiderProjects/crossx/src/CrossX.Services.Identity/Program.cs:line 11

当我添加到webHostBuilder .UseDefaultServiceProvider（options => options.ValidateScopes = false）时，我的问题已解决。但是据我所知，关闭验证并不是一个好主意。另外，当我将AddScope更改为AddTransient时，问题已解决（或至少已运行）。

问题是我不知道在哪里可以找到此错误的来源。我想我不太了解哪里出了问题，因此，如果有人可以帮助我，或者至少提供一个提示，我将不胜感激。

这是我的

Startup.cs：

public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
            services.AddRabbitMq(Configuration);
            services.AddScoped<ICommandHandler<CreateUser>, CreateUserHandler>();       
            services.AddScoped<IEncrypter, Encrypter>();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseMvc();
        }
    }

Program.cs

public class Program
    {
        public static void Main(string[] args)
        {
            ServiceHost.Create<Startup>(args)
                .UseRabbitMq()
                .SubscribeToCommand<CreateUser>()
                .Build()
                .Run();
        }
    }

ServiceHost.cs

public class ServiceHost : IServiceHost
    {
        private readonly IWebHost _webHost;

        public ServiceHost(IWebHost webHost)
        {
            _webHost = webHost;
        }

        public void Run() => _webHost.Run();

        public static HostBuilder Create<TStartup>(string[] args) where TStartup : class
        {
            Console.Title = typeof(TStartup).Namespace;
            var config = new ConfigurationBuilder()
                .AddEnvironmentVariables()
                .AddCommandLine(args)
                .Build();
            var webHostBuilder = WebHost.CreateDefaultBuilder(args)
                .UseConfiguration(config)
//                .UseDefaultServiceProvider(options => options.ValidateScopes = false)
                .UseStartup<TStartup>();

            return new HostBuilder(webHostBuilder.Build());
        }

        public abstract class BuilderBase
        {
            public abstract ServiceHost Build();
        }

        public class HostBuilder : BuilderBase
        {
            private readonly IWebHost _webHost;
            private IBusClient _bus;

            public HostBuilder(IWebHost webHost)
            {
                _webHost = webHost;
            }

            public BusBuilder UseRabbitMq()
            {
                _bus = (IBusClient) _webHost.Services.GetService(typeof(IBusClient));
                return new BusBuilder(_webHost, _bus);
            }

            public override ServiceHost Build()
            {
                return new ServiceHost(_webHost);
            }
        }

        public class BusBuilder : BuilderBase
        {
            private readonly IWebHost _webHost;
            private IBusClient _bus;

            public BusBuilder(IWebHost webHost, IBusClient bus)
            {
                _webHost = webHost;
                _bus = bus;
            }

            public BusBuilder SubscribeToCommand<TCommand>() where TCommand : ICommand
            {
                var handler = (ICommandHandler<TCommand>) _webHost.Services.GetService(typeof(ICommandHandler<TCommand>));
                _bus.WithCommandHandlerAsync(handler);

                return this;
            }

            public BusBuilder SubscribeToEvent<TEvent>() where TEvent : IEvent
            {
                var handler = (IEventHandler<TEvent>) _webHost.Services.GetService(typeof(IEventHandler<TEvent>));
                _bus.WithEventHandlerAsync(handler);

                return this;
            }

            public override ServiceHost Build()
            {
                return new ServiceHost(_webHost);
            }
        }
    }

Answer 1

无法从根提供商处解析作用域服务ICommandHandler<CreateUser>

如错误所述，您不能从根提供者创建作用域服务实例。根提供程序是存在于服务范围之外的根服务提供程序。因此，它无法解析仅应在服务范围内使用的服务。

如果要从根提供者解析作用域服务，例如当您从单例服务中使用它时，应首先使用IServiceScopeFactory创建服务作用域：

var serviceScopeFactory = _webHost.Services.GetService<IServiceScopeFactory>();
using (var scope = serviceScopeFactory.CreateScope())
{
    var handler = (IEventHandler<TEvent>)scope.ServiceProvider.GetService(typeof(IEventHandler<TEvent>))
    // …
}

请注意，服务范围应该是短暂的，您需要在事后将其处置以进行清理。

看您的实现，似乎您将范围服务传递给其他服务以订阅事件。通常这似乎是一个坏主意，因为这意味着在整个应用程序生命周期中，（可能）单例服务将保留对范围服务的引用。这通常是个坏主意（就像我说的那样，作用域服务应该只存在很短的时间）。

您应该问自己，为什么需要在此处对服务进行范围划分，以及是否无法使它们成为单例。或者，如果您实际上需要基于实例的订阅机制（而不是例如仅使用工厂模式或类型的类型）。

无法解析范围服务

1 个答案: