It is important when registering services to do the right thing. There’s six helper methods that will see typical use:

• TryAddSingleton
• TryAddScoped
• TryAddTransient
• AddSingleton
• AddScoped
• AddTransient

Is there only to be one implementation? Then you want a TryAdd* method. Most of the time, you’re only going to need a single implementation, but when you want a collection of services implementing the same interface, the Add* methods are there for you.

Now you need to choose between the available lifetimes:

• Singleton - a single instance will exist for the lifetime of the app. This doesn’t mean that it initializes on registration, but that resolver will only generate one instance.
• Scoped - an instance will be reused for the life of the scope - typically this will be the life of an HttpRequest.
• Transient - every time the resolver will create a new instance. This is the simplest to implement due to the lack of state, but be careful.

.NET 8 adds keyed services, and there’s also some other useful utilities for quickly instantiating services at startup.

References:

• General .NET DI Overview
• .NET DI Guidelines
• ASP.NET DI