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docs: clarify thread safety of service resolution in .NET dependency … #50123

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11 changes: 9 additions & 2 deletions docs/core/extensions/dependency-injection-guidelines.md
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Expand Up @@ -139,9 +139,16 @@ The following third-party containers can be used with ASP.NET Core apps:

## Thread safety

Create thread-safe singleton services. If a singleton service has a dependency on a transient service, the transient service may also require thread safety depending on how it's used by the singleton.
Create thread-safe singleton services. If a singleton service has a dependency on a transient service, the transient service may also require thread safety depending on how it's used by the singleton. The factory method of a singleton service, such as the second argument to [AddSingleton\<TService>(IServiceCollection, Func\<IServiceProvider,TService>)](xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddSingleton%2A), doesn't need to be thread-safe. Like a type (`static`) constructor, it's guaranteed to be called only once by a single thread.

Additionally, the process of **resolving services** from the built-in .NET dependency injection (DI) container is **thread-safe**.
Once an `IServiceProvider` or `IServiceScope` has been built, it's safe to resolve services concurrently from multiple threads.

> [!NOTE]
> Thread safety of the DI container itself only guarantees that constructing and resolving services is safe.
> It doesn't make the resolved service instances themselves thread-safe.
> Any service (especially singletons) that holds shared mutable state must implement its own synchronization logic if accessed concurrently.

The factory method of a singleton service, such as the second argument to [AddSingleton\<TService>(IServiceCollection, Func\<IServiceProvider,TService>)](xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddSingleton%2A), doesn't need to be thread-safe. Like a type (`static`) constructor, it's guaranteed to be called only once by a single thread.

## Recommendations

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