Can I resolve a collection of services of open typed generics with 'GetServices(typeof(IEmpty<>))' in C#?

Question

Having the following interface and their implementations...

public interface IEmpty<T> { }
public class Empty1 : IEmpty<Empty1>{ }
public class Empty2 : IEmpty<Empty2>{ }
public class EmptyN : IEmpty<EmptyN>{ }

allows me to register them and inject them explicitly into constructors

public class NewClass1 {
private IEmpty<Empty1> Empty;
    public NewClass1(IEmpty<Empty1> empty)
    {
        Empty = empty;
    }
    public string EmptyType => $"{Empty.GetType()}";
}

but when I tried to resolve all implementations of 'IEmpty<>' at once...

var allIEmpties = host.Services.GetServices(typeof(IEmpty<>));
allIEmpties.ToList().ForEach(empty => Console.WriteLine(empty.GetType()));

... execution of the code threw a 'NotSupportedException' (Cannot create arrays of open type), which I kind of understand, but leaves me wondering if it can be done and how it would have to be done to get a handle on all Services implementing IEmpty.

Would anyone have an idea of how to achieve this?

My motivation to get this working is to

only register each service once (DRY)
be able to explicity inject services into constructors without the need of some resolver-pattern or named dependencies
load all implementations after startup to validate specific properties they are requried to implement due to the interfaces without having to search reflectively through all assemblies, which is my temporary solution but crawling through a box of needles to find 2 specific ones sounds wrong, if I could have a sorted box offering me direct access to the needles I need...

Using these additional nuget packages:

Microsoft.Extensions.DependencyInjection
Microsoft.Extensions.Hosting
Microsoft.Extensions.Hosting.Abstractions

I have created this proof of concept snippet in LinqPad, in case you'd want to have a go:

void Main()
{
    var args = new List<string>();

    var host = CreateHostBuilder(args.ToArray()).Build();

    var newClass = host.Services.GetService<NewClass1>();
    Console.WriteLine(newClass.EmptyType);

    var oneEmpty = host.Services.GetService(typeof(IEmpty<Empty2>));
    Console.WriteLine(oneEmpty.GetType());

    var allIEmpties = host.Services.GetServices(typeof(IEmpty<>));
    allIEmpties.ToList().ForEach(empty => Console.WriteLine(empty.GetType()));
}

IHostBuilder CreateHostBuilder(string[] args)
{
    var hostBuilder = 
        Host.CreateDefaultBuilder(args)
            .ConfigureAppConfiguration((context, builder) => {
                builder.SetBasePath(Directory.GetCurrentDirectory());
            })
            .ConfigureServices((context, services) => {
                services.AddTransient<IEmpty<Empty1>, Empty1>();
                services.AddTransient<IEmpty<Empty2>, Empty2>();
                services.AddTransient<IEmpty<EmptyN>, EmptyN>();
                services.AddTransient<NewClass1>();
            });
    return hostBuilder;
}

public class NewClass1 {
    private IEmpty<Empty1> Empty;
    public NewClass1(IEmpty<Empty1> empty)
    {
        Empty = empty;
    }
    public string EmptyType => $"{Empty.GetType()}";
}

public interface IEmpty<T> {}
public class Empty1 : IEmpty<Empty1>{ }
public class Empty2 : IEmpty<Empty2>{ }
public class EmptyN : IEmpty<EmptyN>{ }

It looks like a duplicate of [this](https://stackoverflow.com/questions/39174989/how-to-register-multiple-implementations-of-the-same-interface-in-asp-net-core), If you think your question is somehow different, tag me in a comment and I will vote to reopen. — JonasH, Sep 08 '22 at 08:10
hi @JonasH, I could indeed implement a number of workarounds to solve my problem. But I really wonder about resolving `host.GetServices(typeof(IEmpty<>))'. Visual Studio does not complain, so I thought it might be possible... but executing it crashes... Can it be done and if how? <- is not answered by the question you referenced... maybe I should modify and reduce my question to exactly that bit? — Ingo, Sep 08 '22 at 12:33
To me the idea of using generics to facilitate resolution of all implementers of an interface seem ill advised. `IEmpty` is a completely different type from `IEmpty`, and I cannot imagine any DI container supporting resolving from an open generic type. How should the list be represented? the objects do not have any common base type! (except object). Resolving all implementations of an interface on the other hand should be possible, I have done that frequently with UnityContainer, but I have no idea how it is done in the .Net DI container. — JonasH, Sep 08 '22 at 12:58

score 1 · Accepted Answer · answered Sep 09 '22 at 08:48

Resolving a list of generic types based on its open-generic definition by calling GetServices(typeof(IEmpty<>)) is not supported by MS.DI. Although technically possible, there is no DI Container that I'm familiar with that actually supports this.

There are many possible ways to solve your issue. You could, for instance, introduce a non-generic IEmpty (marker) interface that IEmpty<T> inherits from.

You can also go through the code base using Reflection, as you already mentioned, or you can go through the registrations in the ServiceCollection to get all registered IEmpty<T> registrations. This list can than be used to get the list. For instance:

var emptyTypes =
    from s in services
    where s.ServiceType.IsGenericType
    where s.ServiceType.GetGenericTypeDefinition() == typeof(IEmpty<>)
    select s.ServiceType;
    
foreach (Type emptyType in emptyTypes)
{
    var empty = host.Services.GetRequiredService(emptyType);
    Console.WriteLine(empty.GetType());
}

Can I resolve a collection of services of open typed generics with 'GetServices(typeof(IEmpty<>))' in C#?

1 Answers1