Q: Getting multiple service instances for service implementations

[mwpowellhtx]

Hello, consider the following:

public interface ITest
{
}

public class A : ITest { }

public class B : ITest { }

public class C : ITest { }

private static void VerifyServicesOrder()
{
    var services = new ServiceCollection();

    services.AddTransient<ITest, C>();
    services.AddTransient<ITest, B>();
    services.AddTransient<ITest, A>();

    var provider = services.BuildServiceProvider();

    var provided = provider.GetServices<ITest>().ToArray();

    foreach (var p in provided)
    {
        Debug.WriteLine($"Service type {p.GetType().Name}.");
    }
}

It seems as though based on the console output, that services are reported in registration order.

Service type C.
Service type B.
Service type A.

My question is mainly, is that order guaranteed? Are there any cases in which that order might not be so guaranteed?

Thanks...

[mostmand]

As I see in the documentation there is no word for any guarantee. But I was wondering why you may need that. If you give me a clue for your use case I may help with that as I think that the order of service registrations should not be important.

[mwpowellhtx]

I'm not sure what you mean by "clue". Simply I have a family of services, A, B, C, and I want to guarantee their ordering. That's it. You do not need to know beyond that.

As an alternative, perhaps could consider a provider of the set of services. Provided of course I can register to ALL of its implemented interfaces, i.e.

interface IA : ITest { }

class A : IA { }

Anyway, you get my meaning with the above illustration.

[KeterSCP]

As stated in https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection#service-registration-methods:

Services appear in the order they were registered when resolved via IEnumerable<{SERVICE}>.

But personally, I would suggest using keyed services if you can.

[mwpowellhtx]

Keyed is one approach, but is unnecessary, if I can simply register and get based on the types.

Have also considered OrderAttribute, but that is both starting to get a bit intrusive decorating with attirbutes, as well as an unnecessary and I think avoidable maintenance headache.