Mastering Service Registration in Dependency Injection

📖 Introduction

Dependency Injection (DI) is a fundamental design pattern that plays a crucial role in modern software development. It helps decouple dependencies, enhances testability, and allows for flexible service management, making applications more scalable and maintainable.

In traditional programming, classes often instantiate their dependencies directly, leading to tight coupling and making it difficult to modify or test components. DI solves this problem by injecting dependencies externally, allowing components to work independently while promoting separation of concerns.

This article explores different ways to register services in DI containers, helping developers understand how dependencies are managed within an application. Whether you are building a simple console app, an ASP.NET Core API, or a large-scale enterprise application, understanding these techniques will help you make informed design decisions.

By the end of this article, you'll have a clear understanding of the available DI registration approaches, their pros and cons, and best practices to ensure an efficient, well-structured application.

Let's dive in! 🚀

Example Dependencies

public interface IMessageService
{
    void SendMessage (string message);
}

public class EMailService : IMessageService
{
    public void SendMessage (string message) =>
        Console.WriteLine ($"EMail message sent: {message}");
}

public class SMSService : IMessageService
{
    public void SendMessage (string message) =>
        Console.WriteLine ($"SMS message send: {message}");
}

public class MessageProcessor
{
    private readonly IMessageService _messageService;

    public MessageProcessor (IMessageService messageService) =>
        _messageService = messageService;

    public void ProcessMessage (string message) =>
        _messageService.SendMessage (message);
}

Approaches to Service Registration

In this article, we will cover four approaches to service registration:

1. Manual Instance Creation
2. Using Microsoft.Extensions.DependencyInjection (ServiceProvider & ServiceCollection classes)
3. Using IHost & IHostBuilder for DI Management
4. Third-Party DI Containers like Autofac

1️⃣ Manual Instance Creation

Manual DI involves explicitly creating instances of dependencies inside a class. While simple, it leads to tight coupling and can be difficult to scale as applications grow.

Example

// Manual dependency instantiation
IMessageService messageService = new EMailService();

// Calling the method directly
messageService.SendMessage("Hello, Manual Dependency Injection!");

// Constructor injecting service instance into MessageProcessor
MessageProcessor messageProcessorManual = new (messageService);
messageProcessorManual.ProcessMessage("Hello, Manual Dependency Injection! Inject instance as service.");

🌟 Advantages

• Simple and easy to understand.
• No external dependencies or frameworks required.
• Gives direct control over dependency creation.

❌ Disadvantages

• Leads to tight coupling, making future changes difficult.
• Harder to manage dependencies in large applications.
• Testing becomes challenging, as dependencies can't be easily mocked.

📌 Use Case

• Best for small applications or proof-of-concept implementations.
• Suitable for one-off scripts where DI is not needed.
• Useful in early development stages before a full DI system is required.

2️⃣ Using Microsoft.Extensions.DependencyInjection NuGet Package

.NET provides built-in DI support via ServiceCollection & ServiceProvider classes. This method is flexible and integrates well with ASP.NET Core and allows automatic dependency resolution.

Example

// Required NuGet package: Microsoft.Extensions.DependencyInjection

// Registering dependencies
ServiceProvider serviceProvider = new ServiceCollection()
    .AddSingleton<IMessageService, EMailService>()
    .AddSingleton<MessageProcessor>()
    .BuildServiceProvider();

// Resolving services
MessageProcessor messageProcessor = serviceProvider.GetRequiredService<MessageProcessor>();
messageProcessor.ProcessMessage("Hello, Dependency Injection!");

🌟 Advantages

• Built-in support for .NET & ASP.NET Core applications.
• Automatic dependency resolution without manual instantiation.
• Scalable approach for managing dependencies in large projects.

❌ Disadvantages

• Requires understanding DI configuration & service lifetimes.
• Limited features compared to advanced third-party containers.
• Can introduce slight performance overhead if misconfigured.

📌 Use Case

• Best for ASP.NET Core applications where DI is widely used.
• Ideal for scalable architectures with complex dependency chains.
• Suitable for teams using .NET's standard DI system.

3️⃣ Using IHost & IHostBuilder for DI Management

IHost provides a structured way to manage dependencies, logging, and application lifetimes. It is widely used in ASP.NET Core and worker services.

Example

// Required NuGet packages: Microsoft.Extensions.Hosting, Microsoft.Extensions.DependencyInjection

IHost host = Host.CreateDefaultBuilder()
    .ConfigureServices((context, service) =>
    {
        service.AddSingleton<IMessageService, EMailService>();
        service.AddSingleton<MessageProcessor>();
    })
    .Build();

// Resolving services
MessageProcessor messageProcessorHost = host.Services.GetRequiredService<MessageProcessor>();
messageProcessorHost.ProcessMessage("Hello, Dependency Injection with IHost!");

🌟 Advantages

• Provides structure for managing application lifecycles & configuration.
• Consistent with how DI is handled in ASP.NET Core.
• Supports hosted services and background tasks.

❌ Disadvantages

• Adds complexity, especially for simple console applications.
• Slight overhead due to extra structure & service management.

📌 Use Case

• Best for ASP.NET Core applications, worker services, and background tasks.
• Ideal for applications requiring structured lifetime management.
• Suitable when integrating logging, configuration, and hosted services.

4️⃣ Third-Party DI Containers (Autofac, Ninject, SimpleInjector)

Third-party DI frameworks like Autofac provide advanced dependency management, including module registration, property injection, and lifetime scopes.

Example

// Required NuGet package: Autofac
ContainerBuilder builder = new();

// Registering services
builder.RegisterType<EMailService>().As<IMessageService>();
builder.RegisterType<MessageProcessor>();

IContainer container = builder.Build();
using ILifetimeScope scope = container.BeginLifetimeScope();

// Resolving services
MessageProcessor messageProcessorAutofac = scope.Resolve<MessageProcessor>();
messageProcessorAutofac.ProcessMessage("Hello, Autofac Dependency Injection!");

🌟 Advantages

• Supports advanced features like module-based configurations.
• Highly flexible & customizable dependency management.
• Provides fine-grained control over dependency lifetimes.

❌ Disadvantages

• Adds extra dependencies to the project.
• Steeper learning curve compared to built-in DI frameworks.
• May require additional effort to integrate with .NET Core.

⚡ Comparison Summary

Approach	Pros	Cons
Manual Dependency Injection	Simple & direct	Not scalable, hard to manage
Microsoft.Extensions.DependencyInjection	Integrated with .NET	Limited features, not for all scenarios
IHost & IHostBuilder	Provides structured DI	Adds complexity
Third-Party Containers (Autofac, etc.)	Advanced features & flexibility	Extra dependencies, steeper learning curve

🔎 Service Lifetimes in DI

DI manages object lifetimes using three key scopes:

1️⃣ Transient – A new instance is created every time it's requested. Best for lightweight, stateless services.
2️⃣ Scoped – A single instance is maintained per request scope. Ideal for services that need to maintain state within a request.
3️⃣ Singleton – A single instance is shared across the application. Suitable for shared resources or services that are expensive to create.

Example

services.AddTransient<IMessageService, EMailService>();  // Creates a new instance each time
services.AddScoped<IMessageService, EMailService>();     // One instance per request scope
services.AddSingleton<IMessageService, EMailService>();  // Single instance shared across app

⚠ Common Pitfalls in Dependency Injection

While Dependency Injection simplifies service management, improper use can lead to performance bottlenecks, unnecessary complexity, or poor architectural decisions. Below are common mistakes to avoid, followed by best practices to follow:

1️⃣ Overusing Singleton Dependencies

• Issue: Using too many singletons can cause unintended memory leaks or thread-safety issues.
• Example: A singleton logging service might store state unexpectedly.
• Solution: Ensure singletons are truly stateless or use locks for thread safety.

2️⃣ Injecting Too Many Dependencies

• Issue: Having too many constructor parameters can indicate poor design.
• Example: A service needing 10 different injected dependencies might need refactoring.
• Solution: Consider grouping related dependencies into helper classes or using composition.

3️⃣ Misconfiguring Dependency Lifetimes

• Issue: Using singleton dependencies for stateful objects can lead to unexpected behavior.
• Example: A database connection should not be a singleton.
• Solution: Use Scoped lifetime for objects tied to request scope and Transient for short-lived objects.

4️⃣ Using the Service Locator Pattern

• Issue: Instead of injecting dependencies, developers sometimes retrieve them via a global service locator, leading to hidden dependencies and tightly coupled code.
• Example: Using ServiceLocator.Resolve<IMessageService>() hides the dependency, making code harder to test and maintain compared to direct constructor injection.
• Solution: Avoid the service locator pattern. Prefer explicit dependency injection via constructors or method parameters for better testability and maintainability.

5️⃣ Not Disposing Dependencies Properly

• Issue: Forgetting to dispose of IDisposable services causes resource leaks.
• Example: Not disposing DbContext in an EF Core application.
• Solution: Use Scoped dependencies for disposable services or implement IAsyncDisposable.

✅ Best Practices in Dependency Injection

• Choose the Right Lifetime Scope – Use Transient for lightweight services, Scoped for request-specific services, and Singleton for shared dependencies.
• Keep Dependencies Minimal – Reduce unnecessary injections by grouping related dependencies into cohesive service classes.
• Use Constructor Injection Over Property Injection – Constructor injection ensures dependencies exist when the object is created.
• Use IoC Containers Efficiently – Avoid re-registering services, keep single responsibility, and ensure proper dependency resolution.
• Test Dependencies with Mocking Frameworks – Use Moq or FakeItEasy to create mock dependencies instead of relying on real implementations.

📌 Conclusion

Dependency Injection is a powerful technique that significantly improves code organization, modularity, and testability. By injecting dependencies externally, developers reduce coupling, making applications easier to modify and scale.

This article explored multiple DI registration approaches, including:

• Manual Instance Creation, which works for small applications but lacks scalability.
• Microsoft.Extensions.DependencyInjection, offering seamless integration with .NET applications.
• IHost & IHostBuilder, adding structure for managing dependencies efficiently in ASP.NET Core & worker services.
• Third-party DI containers like Autofac, providing advanced features for complex applications.

Choosing the right approach depends on application complexity, performance considerations, and scalability needs. By following best practices, avoiding common pitfalls, and applying proper dependency lifetimes, developers can build robust, maintainable software that adheres to industry standards.

As you continue exploring DI, experiment with different registration methods, test out IoC containers, and refine your implementation strategies to achieve cleaner, more efficient software architectures.

📝 Key Takeaways

• DI decouples components and improves testability, maintainability, and scalability.
• Manual DI is simple but not scalable; use it only for small or throwaway projects.
• Microsoft.Extensions.DependencyInjection is the standard for .NET and ASP.NET Core, balancing simplicity and power.
• IHost & IHostBuilder add structure and are ideal for ASP.NET Core and background services.
• Third-party containers (like Autofac) offer advanced features for complex scenarios.
• Choose the right service lifetime (Transient, Scoped, Singleton) for each dependency.
• Avoid common pitfalls: overusing singletons, injecting too many dependencies, misconfiguring lifetimes, using service locators, and forgetting to dispose resources.
• Follow best practices: keep dependencies minimal, prefer constructor injection, and use mocking frameworks for testing.

📖 Further Reading

• Microsoft Docs: Dependency Injection in .NET
• Autofac Documentation
• ASP.NET Core Fundamentals
• Service Lifetimes Explained
• Mark Seemann: Dependency Injection Principles, Practices, and Patterns

About the Author

Hi, I’m Jiten Shahani, a passionate developer with a strong background in API development and C# programming. Although I’m new to .NET, my journey into learning ASP .NET Core began in December 2024, driven by a desire to build scalable and maintainable applications.

Through my exploration of dependency injection, I’ve realized how crucial it is for writing flexible, testable, and loosely coupled code. This article is a reflection of my learning process, aimed at helping fellow developers, especially beginners, understand the different ways to inject dependencies effectively in C#.

Feel free to connect with me to exchange ideas and learn together!