Singleton vs Scoped vs Transient in .NET: Best Practices & Common Pitfalls Explained

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, practical, and humane way.

1️⃣ 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.

Each lifetime has its use cases, and mixing them without understanding their behavior can lead to unexpected bugs. Now, let’s examine how they interact when injected into each other.

2️⃣ Relationship Table: What Works and What’s Risky?

Injected Service ↓ Inside Singleton Inside Scoped Inside Transient Singleton ✅ Safe ✅ Safe ✅ Safe Scoped ⚠️ Risky ✅ Safe ✅ Safe Transient ⚠️ Risky ✅ Safe ✅ Safe Now, let's break these down with real-world examples and explanations.

3️⃣ Singleton Inside Transient: Is It Possible? ✅

✔️ 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 Code

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! ✅

4️⃣ Singleton Inside Scoped: Is It Possible? ✅

✔️ 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! ✅

5️⃣ Singleton Inside Singleton: Is It Possible? ✅

✔️ 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. ✅

6️⃣ Scoped Inside Singleton: Risky! ⚠️

⚠️ 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 (Problem Example)

public class SingletonService
{
    private readonly IScopedService _scopedService; // ❌ This is dangerous

    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.

7️⃣ Transient Inside Singleton: Risky! ⚠️

⚠️ Another tricky case!

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

❌ Bad Code (Problem Example)

public class SingletonService
{
    private readonly ITransientService _transientService; // ❌ Transient will be reused forever

    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. ✅

8️⃣ Final Summary & Best Practices

🚀 What’s Safe?

✅ Singleton inside anything

✅ Scoped inside Scoped

✅ Transient inside anything (except Singleton without factory)

⚠️ What’s Risky?

❌ Scoped inside Singleton (use IServiceProvider)

❌ Transient inside Singleton (use IServiceProvider)

9️⃣ Conclusion: 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.

✔️ 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! 🚀