While preparing a training about .NET Core I stumbled over a feature I was unaware it existed in ASP.NET Core: Scope Validation.
One of the nice things inside ASP.NET Core is that it has built-in support for Dependency Injection.
Service lifetime
When registering your dependencies you have 3 service lifetimes to choose from:
- Transient: Transient lifetime services are created each time they're requested. This lifetime works best for lightweight, stateless services.
- Scoped: Scoped lifetime services are created once per request.
- Singleton: Singleton lifetime services are created the first time they're requested (or when
ConfigureServicesis run and an instance is specified with the service registration). Every subsequent request uses the same instance.
Simple so far. The problem is that you can shoot yourself in the foot when you start combining multiple lifetimes in the same object tree.
For example when using a scoped service in a middleware, and you inject it via constructor injection, the service will behave as a singleton as the service is injected only once at the construction of the middleware. (Note: Inject the scoped service into the Invoke or InvokeAsync method as a solution)
Another example where it can go wrong is when trying to resolve a scoped service from a singleton. This can lead to an incorrect state in your application.
To prevent this kind of problems from happening, Microsoft introduced Scope validation.
Scope validation
If you are using the default WebHost builder scope validation is already enabled as one of the steps when executing CreateDefaultBuilder. Behind the scenes it sets ServiceProviderOptions.ValidateScopes to true if the app's environment is Development.
When ValidateScopes is set to true, the default service provider performs checks to verify that:
- Scoped services aren't directly or indirectly resolved from the root service provider.
- Scoped services aren't directly or indirectly injected into singletons.
The root service provider is created when BuildServiceProvider is called. The root service provider's lifetime corresponds to the app/server's lifetime when the provider starts with the app and is disposed when the app shuts down.
Scoped services are disposed by the container that created them. If a scoped service is created in the root container, the service's lifetime is effectively promoted to singleton because it's only disposed by the root container when app/server is shut down. Validating service scopes catches these situations.
Remark: if you want to validate scopes for all environment, you can configure the ServiceProviderOptions with UseDefaultServiceProvider on the host builder: