Question: Multiple calls to AddSingleton<TInterface,TImplementation>

[kimbell]

This question is inspired by some exploring with IHostedService.

My understanding has been that the last call to AddSingleton() replaces previously registered information.

Lets say the framework calls AddSingleton<IMemoryCache,MemoryCache>() based on an AddXYZ() method.
I want my own implementation, so if I add AddSingleton<IMemoryCache,MyMemoryCache>() after the framework, I get my implementation.

When playing with IHostedService, I discovered that I could add multiple services

AddSingleton<IHostedService, MyHostedService1>()
AddSingleton<IHostedService, MyHostedService2>()

Both services get activated, indicating that the DI system keeps track of all the registered implementations.

How does the DI system know what implementation to use if I have an object that requires a single IHostedService? The HostedServiceExecutor being used by the framework takes an IEnumerable<IHostedService>.

Can we then deduce that if a single reference is needed, it takes the last one registered.
If it requests an IEnumerable, we get everything?

If we take this one step further, and decide to use a different DI implementation. Is this type of behavior 'standard' in the other popular implementations? If one chooses the wrong DI system, it could have a negative effect on hosted services.

[davidfowl]

Its about the consumption. The current contract for the IServiceCollection is as you describe. If you add multiple services and the consumer asks for a list, it'll manifest all of them. If you ask for a single one, then you'll get the last one.

If we take this one step further, and decide to use a different DI implementation. Is this type of behavior 'standard' in the other popular implementations? If one chooses the wrong DI system, it could have a negative effect on hosted services.

At the moment it's a requirement for all of the adapters (it's part of the specification tests) so if you use an adapter for Microsoft.Extensions.DependendencyInjection that passes the tests behaves like this.

There's a much longer discussion here about the requirements in general #433.

[kimbell]

Thanks, learn something every day :)