C# Dependency Injection - injecting interface that takes the same interface as a parameter

Question

I've run into a bit of trouble trying to use dependency injection and need help.

I have a service IService that is implemented a few different ways in my application.

ServiceA : IService { public ServiceA(IDependencyA A, IDependency B) {...} }
ServiceB : IService { public ServiceB(IDependencyA A, IDependency B) {...} }
ServiceC : IService { public ServiceC(IService serviceA, IService serviceB) {...} }

In my startup.cs file, I will pick which one to use based on a parameter in my configuration file. Something like this:

var service = Configuration["AppConfig:Service:ID"];
switch(service) {
    case "A":
        services.AddTransient<IService, ServiceA>();
    case "B":
        services.AddTransient<IService, ServiceB>();
    case "C":
        // ??
}

I am able to create and use service A and service B very easily. I inject their dependencies A and B before hand and they get created just fine. The problem is with the third service. I need to inject the two other services into it. My question is: what is the best to do this?

Is there a way I can create concrete implementations of service A and B but somehow use the injected dependency A and B in their constructor? Do I have to mess around with the interfaces to get this to work? Maybe I have to change the constructor of ServiceC to take in concrete implementations of A & B?

Updated: don't know if it's the best solution, but I ended up doing the following to get it to work.

...
case "C":
    services.AddTransient<ServiceA>();
    services.AddTransient<ServiceB>();
    services.AddTransient<IService>(s => 
        new ServiceC(
            s.GetService<ServiceA>(),
            s.GetService<ServiceB>()
    ));

Add more details about service C. If it really needs these 2 concrete implementations, then you should change the constructor. Otherwise it will break Liskov Substitution Principle. Fix switch, there are no breaks, and serviceA is registered 2 times. — apocalypse
– apocalypse, Commented Sep 1, 2020 at 19:35
The goal of service C is to use the implementation of service A, but if service A could not provide a correct result, fallback to the result of service B. — mymemesarespiciest
– mymemesarespiciest, Commented Sep 1, 2020 at 19:37
Create a factory pattern for required services, and instantiate them when needed. Not everything needs to be handles by DI() — monstertjie_za
– monstertjie_za, Commented Sep 1, 2020 at 19:39
Based on your last comment here the usage of services based on their result, you could achieve this in a cleaner fashion using a concept similar to the Strategy Pattern. It certainly would be better if you could predict whether or not the result will be successful. — ColinM
– ColinM, Commented Sep 1, 2020 at 20:12
@mymemesarespiciest even if you don't use Polly directly you should see how it works, how it solves the same problems you may have — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Sep 2, 2020 at 10:10

Jeremy Lakeman · Accepted Answer · 2020-09-04 06:26:11Z

1

Another alternative. Introduce another interface, so you can register the both ServiceC and it's dependencies at the same time, without any ambiguity or circular references.

interface IService {}
interface IServiceImpl : IService {}

ServiceA : IServiceImpl { public ServiceA(IDependencyA A, IDependency B) {...} }
ServiceB : IServiceImpl { public ServiceB(IDependencyA A, IDependency B) {...} }
ServiceC : IService { public ServiceC(IEnumerable<IServiceImpl> services) {...} }

services.AddTransient<IServiceImpl, ServiceA>();
services.AddTransient<IServiceImpl, ServiceB>();
services.AddTransient<IService, ServiceC>();

answered Sep 4, 2020 at 6:26

Jeremy Lakeman

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5 Comments

Jeremy Lakeman Over a year ago

Yes it will. DI will usually inject the first registered type. Note that I changed the constructor of ServiceC to IEnumerable<T> so it will receive all of the service implementations.

Roar S. Over a year ago

I understand why ServiceC is OK. I need to test setup for ServiceA and ServiceB on my own during this weekend.

Roar S. Over a year ago

BTW: When testing this, how should I resolve ServiceA and ServiceB in e.g. a controller? By injecting IServiceImpl?

Jeremy Lakeman Over a year ago

The idea was to keep IService as the "public API", and only refer to IServiceImpl "internally" as an implementation detail...

Roar S. Over a year ago

I see that OP solved it by using services.AddTransient<ServiceA>(); services.AddTransient<ServiceB>(); , maybe no point digging further into this for now. Anyway, Now I do understand the logic behind your answer, +1 for that :-) Thanks for your time Jeremy. BR

Iqan Shaikh · Accepted Answer · 2020-09-01 19:29:05Z

0

There must be a better explanation for having such dependencies.

Microsoft.Extensions.DependencyInjection does not provide named registrations like other IoC containers i.e. Castle.Windsor.

There is a way you can still register ServiceC with instances of A and B

            services.AddTransient<ServiceA>();
            services.AddTransient<ServiceB>();
            services.AddTransient<IService>(serviceProvider => {
                return new ServiceC((IService)serviceProvider.GetRequiredService(typeof(ServiceA)), (IService)serviceProvider.GetRequiredService(typeof(ServiceB)));
            });

answered Sep 1, 2020 at 19:29

Iqan Shaikh

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2 Comments

mymemesarespiciest Over a year ago

Funny that you post this because it's nearly identical to the solution I just found that ended up working. The only difference is that you don't need cast to IService because those services are already implementing that interface.

Iqan Shaikh Over a year ago

aha nice! Btw. I dont refresh while writing. and "funny" enough you think like that :) The only reason for casting is GetRequiredService does require typecasting

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