Singleton Dependencies in Scoped Services | ASP.NET Core DI Pattern #44

Learn how to safely inject singleton services into scoped services in ASP. NET Core dependency injection and understand the lifetime relationship in . NET 10.

✅ WHAT YOU'LL LEARN:

How singleton services work when injected into scoped services and why this pattern is completely safe without captive dependency issues.

Common use cases where scoped services need singleton dependencies and best practices for thread safety considerations.

🎯 HOW IT WORKS:

Scoped services are created once per scope while singleton dependencies are created once for the application lifetime. All scoped instances across all scopes share the same singleton instance.

✨ COMMON USE CASES:

Scoped repositories using singleton configuration - DbContext with singleton logging - Scoped services accessing singleton caches - Business logic with singleton settings - Scoped handlers using singleton HTTP clients - Services needing singleton utilities.

🔒 THREAD SAFETY REQUIREMENTS:

Singleton services must be thread-safe since they're shared across all scopes and requests. Multiple scoped instances from different requests might access the singleton concurrently. Use immutable singletons whenever possible.

⚡ PERFORMANCE BENEFITS:

Shared singleton resources reduce memory and creation overhead. Configuration objects - loggers - caches - and HTTP clients are efficiently reused across all scoped services.

⚠️ CRITICAL WARNING:

Never store request-specific or scope-specific state in singletons. Singletons are shared across ALL requests and scopes. Any mutable state must be properly synchronized for concurrent access.

🔧 BEST PRACTICES:

Design singletons as immutable when possible - use thread-safe collections for mutable state - avoid ambient context in singletons - clearly document thread safety requirements.

🎓 Full Course Playlist: https://www.youtube.com/watch?v=OlRBdXaQm2M&list=PLaFzfwmPR7_KtNJZt9pn8RtDAA-Muk4ql

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