Dependency Injection in .NET: Service Lifetimes

Using the wrong DI lifetime is the #1 cause of concurrency bugs in ASP.NET Core. We revisit Singleton, Scoped, and Transient with a focus on thread safety.

The Three Lifetimes

The Captive Dependency Problem

Injecting a Scoped service into a Singleton service is a fatal error. The Scoped service will stay alive forever (captured), possibly holding onto a DbConnection.

// ❌ BAD
public class SingletonService
{
    private readonly ScopedService _scoped; // Captured!
    public SingletonService(ScopedService scoped) => _scoped = scoped;
}

// ✅ GOOD (IServiceScopeFactory)
public class SingletonService
{
    private readonly IServiceScopeFactory _scopeFactory;
    
    public void DoWork()
    {
        using var scope = _scopeFactory.CreateScope();
        var scoped = scope.ServiceProvider.GetRequiredService<ScopedService>();
        scoped.DoThing();
    }
}

Key Takeaways

• EF Core `DbContext` is **Scoped**. Never inject it into a Singleton.
• Use `ValidateScopes = true` in development to catch these bugs early.