Understanding Dependency Injection Lifetimes in ASP.NET Core: Transient vs Scoped vs Singleton

Let's illustrate the differences between AddTransient, AddScoped, and AddSingleton services with examples:

1. AddTransient:

   csharp
   services.AddTransient<IMyService, MyService>();
   

   In this example, every time IMyService is requested, a new instance of MyService is created. Each usage or request will receive its own independent instance of the service. Transient services are suitable when you want a fresh instance every time, such as for stateful services that store request-specific data.

2. AddScoped:

   csharp
   services.AddScoped<IMyService, MyService>();
   

   In this case, a single instance of MyService is created for the scope of a web request. Any component within that web request will receive the same instance of MyService. However, subsequent web requests will have their own separate instance. Scoped services are useful when you need to share the same instance within a specific unit of work, such as processing a web request.

3. AddSingleton:

   csharp
   services.AddSingleton<IMyService, MyService>();
   

   Here, a single instance of MyService is created and shared across the entire application. All requests and components will receive and share the same instance of MyService. Singleton services are suitable for stateless services or for cases where you want a single, globally accessible instance, such as for application-wide configuration settings.

To further illustrate, let's consider an example where IMyService represents a logging service:

csharp
public interface IMyService
{
    void Log(string message);
}

public class MyService : IMyService
{
    public void Log(string message)
    {
        // Log the message
    }
}

Now, if you register the service with different lifetimes:

csharp
// Transient
services.AddTransient<IMyService, MyService>();

// Scoped
services.AddScoped<IMyService, MyService>();

// Singleton
services.AddSingleton<IMyService, MyService>();

In a scenario where multiple components or requests require the logging service:

csharp
public class MyComponent
{
    private readonly IMyService _myService;

    public MyComponent(IMyService myService)
    {
        _myService = myService;
    }

    public void DoSomething()
    {
        _myService.Log("Doing something...");
    }
}

1. With AddTransient, each MyComponent instance will receive a new and separate instance of MyService when it is resolved.
2. With AddScoped, all MyComponent instances within the same web request will share the same instance of MyService, while different web requests will have their own separate instances.
3. With AddSingleton, all MyComponent instances across all requests will share the same instance of MyService.

These differences in behavior allow you to control how instances of services are created, shared, and scoped within your ASP.NET Core application based on your specific requirements and scenarios.