.NetCore Autofac.Core.DependencyResolutionException: Circular component dependency detected

依赖注入实现类循环引用导致错误

一次拉取代码之后编译、启动未出现错误，服务启动后调用API报错，错误信息如下：

.......
An unandled exception has occurred while executing the request.
Autofac.Core.DependencyResolutionException: An exception was thrown while activating
 UOM.Service.DomainService.MachineDomainService -> 
 UOM.Service.DomainService.ServiceNodeDomainService -> 
 UOM.Service.DomainService.ServiceDomainService.
 ---> Autofac.Core.DependencyResolutionException: Circular component dependency 
 detected: UOM.Service.DomainService.MachineDomainService -> 
 UOM.Service.DomainService.ServiceNodeDomainService -> 
 UOM.Service.DomainService.ServiceDomainService -> 
 UOM.Service.DomainService.ServiceNodeDomainService.
   at Autofac.Core.Resolving.CircularDependencyDetector.CheckForCircularDependency(IComponentRegistration registration, Stack`1 activationStack, Int32 callDepth)
   at Autofac.Core.Resolving.ResolveOperation.GetOrCreateInstance(ISharingLifetimeScope currentOperationScope, IComponentRegistration registration, IEnumerable`1 parameters)
   at Autofac.Core.Resolving.InstanceLookup.ResolveComponent(IComponentRegistration registration, IEnumerable`1 parameters)
   at Autofac.Core.Activators.Reflection.AutowiringParameter.<>c__DisplayClass0_0.<CanSupplyValue>b__0()
   at Autofac.Core.Activators.Reflection.ConstructorParameterBinding.Instantiate()
   at Autofac.Core.Activators.Reflection.ReflectionActivator.ActivateInstance(IComponentContext context, IEnumerable`1 parameters)
   at Autofac.Core.Resolving.InstanceLookup.Activate(IEnumerable`1 parameters, Object& decoratorTarget)
   --- End of inner exception stack trace ---
   at Autofac.Core.Resolving.InstanceLookup.Activate(IEnumerable`1 parameters, Object& decoratorTarget)
   at Autofac.Core.Resolving.InstanceLookup.Execute()
   at Autofac.Core.Resolving.ResolveOperation.GetOrCreateInstance(ISharingLifetimeScope currentOperationScope, IComponentRegistration registration, IEnumerable`1 parameters)
   at Autofac.Core.Resolving.ResolveOperation.ResolveComponent(IComponentRegistration registration, IEnumerable`1 parameters)
   at Autofac.Core.Resolving.ResolveOperation.Execute(IComponentRegistration registration, IEnumerable`1 parameters)
   at Autofac.Core.Lifetime.LifetimeScope.ResolveComponent(IComponentRegistration registration, IEnumerable`1 parameters)
......

错误分析

错误信息解读

1. Autofac.Core.DependencyResolutionException: An exception was thrown while activating
   翻译为激活时抛出异常，意思是无法创建接口的实例
2. Autofac.Core.DependencyResolutionException: Circular component dependency
   翻译为组件形成循环引用

寻找循环

UOM.Service.DomainService.MachineDemainService -> 
UOM.Service.DomainService.ServiceNodeDomainService -> 
UOM.Service.DomainService.ServiceDomainService -> 
UOM.Service.DomainService.ServiceNodeDomainService.

编号
① MachineDomainService
② ServiceNodeDomainService
③ ServiceDomainService

查看服务代码后的确是存在循环引用

反思

在此次事件中，由于没有进行评估和详细设计，直接进行编码，从而没有把类的职责划分清楚，所以导致循环引用。实际上可以设置允许循环引用

builder.RegisterType<UserService>()
 .InstancePerLifetimeScope()
 .PropertiesAutowired(PropertyWiringOptions.AllowCircularDependencies);

这种处理方式只会导致问题变得越来越糟糕，应当解除循环引用和减少耦合度，提高代码的可维护性。如果项目非常紧急可以这样在处理（给自己挖坑）

在实际工作容易出现循环依赖的场景有：

1. 同一个工程不同模块依赖注入；
2. 多个过程进行项目；
3. 多个服务进行通信（Http、MQ、Rpc）等。

保持类、模块、服务的自治性，尽量不要形成依赖。单体服务设计时同一层次不要直接引用，尽量创建自身能够控制的实体，调用方（高层级）将需要依赖的转为该实体传入，降低耦合度。微服务中尽量少用Rpc，避免滥用，如果非要用也必须统一管理。服务数量过多时，是否存在循环依赖则需要进行整体结构的宏观把握，理清楚服务间的关系，进行良好的服务划分，怎么划分服务则需要根据具体业务分析和经验了。