When working with .NET applications, Dependency Injection (DI) is one of the most powerful tools at our disposal. But as soon as we start injecting different types of services, questions arise:

• Can I inject a transient service inside a singleton?
• What happens if I inject a scoped service inside another scoped service?
• Will a transient inside a singleton still be transient?

If you've ever had these doubts, you're not alone! In this post, we'll break down these relationships in a simple, and practical way.

What Are Singleton, Scoped, and Transient?

Before diving into their relationships, let’s refresh our understanding of these service lifetimes in .NET DI:

• Singleton → Created once per application and shared globally.
• Scoped → Created once per request (for APIs, it's one per HTTP request).
• Transient → Created every time it's requested.

When working with dependency injection in .NET, the lifetime of your services can create unexpected behavior, especially when different lifetimes interact. Here’s a quick guide to what combinations are safe and which ones are risky:

• Injecting a Singleton into any service (Singleton, Scoped, or Transient) is safe. Since it's a long-lived instance, it doesn't pose any issues regardless of where it's used.
• Injecting a Scoped Service into another Scoped or Transient service is also safe. They all share the same request context, so they behave as expected.
• However, injecting a Scoped Service into a Singleton is risky. The Singleton outlives the Scoped instance, which could lead to disposed objects being accessed or stale data issues.
• Injecting a Transient Service is generally safe across the board. Whether it’s injected into Singleton, Scoped, or other Transient services, each injection creates a new instance—keeping things isolated.

Now, let's break these down with real-world examples and explanations.

Singleton Inside Transient

Yes, it's completely fine!

• Since Singleton services are created once and shared, every transient instance will receive the same singleton instance.
• Example: A transient FileProcessorService that needs a LoggerService (singleton).

Example:

public class TransientService
{
    private readonly ISingletonService _singletonService;

    public TransientService(ISingletonService singletonService)
    {
        _singletonService = singletonService;
    }

    public void Execute()
    {
        _singletonService.DoWork();
    }
}

What happens here?

Even though multiple transient services are created, they all reuse the same singleton instance. Perfectly fine!

Singleton Inside Scoped

Yes, and it's safe!

• Scoped services are created per request, but they can still use a single global instance of a singleton service.
• Example: A Scoped DatabaseRepository that relies on a CacheService (singleton).

public class ScopedService
{
    private readonly ISingletonService _singletonService;

    public ScopedService(ISingletonService singletonService)
    {
        _singletonService = singletonService;
    }

    public void Execute()
    {
        _singletonService.DoWork();
    }
}

What happens here?

Each request will get its own ScopedService instance, but all instances will use the same SingletonService instance. Works smoothly!

Singleton Inside Singleton

Absolutely!

• A singleton can inject another singleton without any issues.
• Example: A CacheManager singleton that depends on a LoggerService singleton.

public class SingletonService
{
    private readonly IAnotherSingleton _anotherSingleton;

    public SingletonService(IAnotherSingleton anotherSingleton)
    {
        _anotherSingleton = anotherSingleton;
    }
}

Since both services exist for the entire application lifetime, this is a natural and safe dependency. ✅

Scoped Inside Singleton

This is where things get Tricky.

• Since singleton services live forever, but scoped services live only for a request, a singleton might hold onto an expired scoped service.
• This can cause "ObjectDisposedException" errors!

Bad Code (Example)

public class SingletonService
{
    private readonly IScopedService _scopedService;

    public SingletonService(IScopedService scopedService)
    {
        _scopedService = scopedService;
    }
}

If a new request comes in, _scopedService may no longer be valid, leading to errors.

Correct Approach (Use IServiceProvider)

public class SingletonService
{
    private readonly IServiceProvider _serviceProvider;

    public SingletonService(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public void Execute()
    {
        using (var scope = _serviceProvider.CreateScope())
        {
            var scopedService = scope.ServiceProvider.GetRequiredService<IScopedService>();
            scopedService.DoWork();
        }
    }
}

Now, each time Execute() is called, a fresh scoped service is created without lingering issues.

Transient Inside Singleton

Another Tricky case!

If a singleton injects a transient, the transient is created only once, making it behave like a singleton!

Bad Code (Example)

public class SingletonService
{
    private readonly ITransientService _transientService;

    public SingletonService(ITransientService transientService)
    {
        _transientService = transientService;
    }
}

Correct Approach (Use IServiceProvider)

public class SingletonService
{
    private readonly IServiceProvider _serviceProvider;

    public SingletonService(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public void Execute()
    {
        var transientService = _serviceProvider.GetRequiredService<ITransientService>();
        transientService.DoWork();
    }
}

Now, a new transient service is created every time we call Execute(), keeping the transient behavior intact. ✅

Therefore,

Safe Option?

• Singleton inside anything
• Scoped inside Scoped
• Transient inside anything (except Singleton without factory)

Has Risk?

• Scoped inside Singleton (use IServiceProvider)
• Transient inside Singleton (use IServiceProvider)

Inject Smartly

Understanding how these lifetimes interact can save you from tricky bugs in .NET applications.

Whenever in doubt, follow these rules:

• Singleton? Global & shared.
• Scoped? Per request.
• Transient? Fresh every time.

And, avoid injecting Scoped or Transient inside Singleton unless using IServiceProvider.

By applying these best practices, you’ll ensure a clean, efficient, and bug-free dependency injection setup in your .NET apps!

Happy coding!