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Angular reports NG0200: Circular Dependency in DI when a service depends—directly or through other services—on itself. Follow the dependency path back to its starting point, then refactor the services or their communication so Angular can construct them without a cycle. Angular’s NG0200 guide recommends breaking the dependency loop.
What does NG0200 mean?
Angular’s dependency injection system is trying to create a service, but the dependencies required to create it eventually lead back to that same service. For example, if UserService needs EmployeeService and EmployeeService needs UserService, neither can be constructed first. The cycle can also involve more services: A depends on B, B on C, and C on A.
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The error may include a dependency path, such as AuthClient -> UserClient -> AuthClient. Treat the names in your own error as a route to investigate, not as a complete explanation: inspect how each dependency is obtained and what else its construction requires. Angular advises using the call stack to locate the cycle. See the dependency-injection troubleshooting guide.
How to trace the dependency cycle
- Read the full error and call stack. Write down the service or token named in the path, then follow each dependency hop until the chain returns to a service already being created.
- Map indirect dependencies. Check the constructors and injected fields of every service in the chain. A service that appears to depend on only one other service may lead to the cycle through that service’s own dependencies.
- Inspect providers and factories. If constructor dependencies do not explain the path, check provider configuration and factory functions that create or request the affected services.
- Check relevant imports. Angular’s troubleshooting guidance notes that import cycles in module files carrying service dependencies can also contribute. Follow imports connected to the services and providers in the reported path.
- Redraw the graph after changing it. Confirm that the revised dependency arrows no longer lead back to a service being created, then run the application path that triggered the error.
How to fix NG0200
Extract shared logic into a third service
This is the preferred structural fix when two services depend on each other because they share behavior. Move that behavior into a separate service, then have both original services depend on the new service instead of injecting each other. This changes the dependency direction rather than merely delaying the same cycle.
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Use event-based communication when the services should be decoupled
If one service needs to notify another about something that happened, event-based communication—such as a Subject—may fit better than direct mutual injection. Choose this when the relationship is naturally a notification or event, rather than shared functionality that belongs in a common service.
Use Injector.get() only as a last resort
Angular documents Injector.get() as a last-resort way to defer retrieving a dependency. It can make the dependency less visible and shift retrieval to a later point, so first consider whether the services’ responsibilities or communication direction should change.
Does forwardRef() fix a service circular dependency?
No. forwardRef() defers resolving a reference in situations such as declaration order or provider configuration. Angular’s example uses useExisting: forwardRef(() => MenuItem) for a component provider; that is different from breaking a runtime cycle between services. Angular’s troubleshooting guide explicitly advises against using forwardRef() to address service circular dependencies. See the forwardRef() API documentation.
Is lazy loading a general fix for NG0200?
No. Angular’s lazy-services guide describes loading an auto-provided service on demand, often for a large or rarely used feature. Once loaded, Angular resolves the service through its regular dependency-injection system. Lazy loading is not a substitute for diagnosing and removing the cycle.
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NG0200 versus NG0203
NG0200 means the dependency graph loops back to a service being created. NG0203 is a different error: it occurs when inject() is called outside a valid injection context. Angular documents valid contexts including DI-managed construction, field initializers, and provider or InjectionToken factories. If the message is NG0203, follow Angular’s injection-context guide rather than looking for a circular service dependency.
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