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FakeItEasy: how to unit test abstract class

I want to mock abstract class public method to returns different values. Is it doable using fakeiteasy?
public class Worker: IInterface
{
private readonly ILogger _logger;
provate readonly IServiceProvider _serviceProvider;
public Worker(ILogger logger, IServiceProvider provider)
{
_logger = logger;
_serviceProvider = provider;
}
public Handler(Order order)
{
var service = _serviceProvier.GetService(order.Properties.Type);
service.DoTaskAsync(...);
}
}
public abstract class ServiceBases
{
public async Task DoTaskAsync(...);
}
[Fact]
public async Task WorkerHandler_Return_Success()
{
// how to mock ServiceBases.DoTaskAsync to returns null or throw exception ?
}

This is complicated because you've got two things to figure out:
How to create a mock of your abstract class
How to set up your IServiceProvider to return an instance of the class that inherits from the abstract class.
Let's start with mocking the abstract class. The easiest way to do that is to create a class that implements the abstract class. You could do this:
public class FakeServiceBases : ServiceBases
{
public int NumberOfTimesCalled { get; private set; }
public override async Task DoTaskAsync()
{
NumberOfTimesCalled++;
}
}
Presumable you're mocking this so that you can make sure the class that depends on this calls DoTaskAsync(). If you create an instance of this fake class then when Handler calls it, it will increment the value. When the test runs you can check that the value has been incremented.
Or maybe you want to make sure that the expected arguments were passed. I don't know what the arguments are, but when DoTaskAsync() is called you can add the arguments to a list instead of incrementing a number, like this:
public class FakeServiceBases : ServiceBases
{
public List<SomeArgumentType> CallsToDoAsync { get; } = new List<SomeArgumentType>();
public override async Task DoTaskAsync(SomeArgumentType someArgument)
{
CallsToDoAsync.Add(someArgument);
}
}
Same thing. After calling your Handler method you can inspect the arguments that were passed to service.DoTaskAsync by looking at what's in the CallsToDoAsync list.
That takes care of mocking ServiceBases. The other step is getting your IServiceProvider to always return a FakeServiceBase regardless of the type passed to it.
That you can do with FakeItEasy.
var fakeServiceBase = new FakeServiceBases();
var serviceProvider = A.Fake<IServiceProvider>();
A.CallTo(() => serviceProvider.GetService(A<Type>.Ignored)).Returns(fakeServiceBase);
var testSubject = new Worker(NullLogger.Instance, serviceProvider);
This creates a faked IServiceProvider. Whenever GetService is called, regardless of the type passed to it, it will return the instance of FakeServiceBases that you created. After calling methods on the Worker you can inspect fakeServiceBase to make sure methods were called on it like you expected.
We can make this class easier to test by removing the dependency on IServiceProvider.
Let's change Worker to look like this:
public class Worker
{
private readonly ILogger _logger;
private readonly Func<Type, ServiceBases> _getServicesBases;
public Worker(ILogger logger, Func<Type, ServiceBases> getServicesBases)
{
_logger = logger;
_getServicesBases = getServicesBases;
}
public async Task Handler(Order order)
{
// Calls the function - passes a Type and gets a ServiceBases
var service = _getServicesBases(order.Properties.Type);
service.DoTaskAsync(...);
}
}
Now it doesn't depend on IServiceProvider. It depends on a Func<Type, ServiceBases>. In other words, it depends on a function that will take a Type and return an object that implements ServicesBases.
When you're registering your types in your Startup you can do this:
services.AddSingleton<Func<Type, ServiceBases>>(serviceProvider =>
new Func<Type, ServiceBases>(type => (ServiceBases)serviceProvider.GetService(type)));
So at runtime the function will actually resolve the type from the IServiceProvider.
But now that your class depends on a function, the mock is even simpler. You don't need to mock an IServiceProvider. Instead, you provide a "real" function that returns your FakeServiceBases:
var fakeServiceBase = new FakeServiceBases();
Func<Type, ServiceBases> getServiceBases = type => fakeServiceBase;
var testSubject = new Worker(NullLogger.Instance, getServiceBases);

Creating an instance of generic class, how to easily set the type to the calling class

I have a generic class LoggerHelper<T>. There are properties in many different classes in the application, whose values are to be set to instances of LoggerHelper. In the constructors of these classes, this property will be set. For example:
public class Importer
{
ILogger _logger;
public Importer()
{
this._logger = new LoggerHelper<Importer>();
}
}
How can I avoid specifying the type each time? Is there are suitable pattern for this?

You could use a factory method, thus a type could be inferred by the compiler.
Static method
public static class LoggerHelperFactory
{
public static LoggerHelper<T> Create<T>(T instance)
{
return new LoggerHelper<T>();
}
}
Then you can call it as:
_logger = LoggerHelperFactory.Create(this);
Extension method
public static class LoggerHelperFactoryExtensions
{
public static LoggerHelper<T> CreateLogger<T>(this T instance)
{
return new LoggerHelper<T>();
}
}
Usage:
_logger = this.CreateLogger();

.NET Core multiple IDbConnection implementation via AddTransient [duplicate]

I have services that are derived from the same interface.
public interface IService { }
public class ServiceA : IService { }
public class ServiceB : IService { }
public class ServiceC : IService { }
Typically, other IoC containers like Unity allow you to register concrete implementations by some Key that distinguishes them.
In ASP.NET Core, how do I register these services and resolve them at runtime based on some key?
I don't see any Add Service methods that take a key or name parameter, which would typically be used to distinguish the concrete implementation.
public void ConfigureServices(IServiceCollection services)
{
// How do I register services of the same interface?
}
public MyController:Controller
{
public void DoSomething(string key)
{
// How do I resolve the service by key?
}
}
Is the Factory pattern the only option here?
Update1
I have gone though the article here that shows how to use the factory pattern to get service instances when we have multiple concrete implementations. However, it is still not a complete solution. When I call the _serviceProvider.GetService() method, I cannot inject data into the constructor.
For example consider this:
public class ServiceA : IService
{
private string _efConnectionString;
ServiceA(string efconnectionString)
{
_efConnecttionString = efConnectionString;
}
}
public class ServiceB : IService
{
private string _mongoConnectionString;
public ServiceB(string mongoConnectionString)
{
_mongoConnectionString = mongoConnectionString;
}
}
public class ServiceC : IService
{
private string _someOtherConnectionString
public ServiceC(string someOtherConnectionString)
{
_someOtherConnectionString = someOtherConnectionString;
}
}
How can _serviceProvider.GetService() inject the appropriate connection string?
In Unity, or any other IoC library, we can do that at type registration. I can use IOption, however, that will require me to inject all settings. I cannot inject a particular connection string into the service.
Also note that I am trying to avoid using other containers (including Unity) because then I have to register everything else (e.g., Controllers) with the new container as well.
Also, using the factory pattern to create service instances is against DIP, as it increases the number of dependencies a client has details here.
So, I think the default DI in ASP.NET Core is missing two things:
The ability to register instances using a key
The ability to inject static data into constructors during registration

I did a simple workaround using Func when I found myself in this situation.
Firstly declare a shared delegate:
public delegate IService ServiceResolver(string key);
Then in your Startup.cs, setup the multiple concrete registrations and a manual mapping of those types:
services.AddTransient<ServiceA>();
services.AddTransient<ServiceB>();
services.AddTransient<ServiceC>();
services.AddTransient<ServiceResolver>(serviceProvider => key =>
{
switch (key)
{
case "A":
return serviceProvider.GetService<ServiceA>();
case "B":
return serviceProvider.GetService<ServiceB>();
case "C":
return serviceProvider.GetService<ServiceC>();
default:
throw new KeyNotFoundException(); // or maybe return null, up to you
}
});
And use it from any class registered with DI:
public class Consumer
{
private readonly IService _aService;
public Consumer(ServiceResolver serviceAccessor)
{
_aService = serviceAccessor("A");
}
public void UseServiceA()
{
_aService.DoTheThing();
}
}
Keep in mind that in this example the key for resolution is a string, for the sake of simplicity and because OP was asking for this case in particular.
But you could use any custom resolution type as key, as you do not usually want a huge n-case switch rotting your code. Depends on how your app scales.

DISCLAIMER: this is not a good solutions for all fits. I reach it to solve a specific problem. in the comments below you can read and evaluate the drawbacks of this approach.
Another option is to use the extension method GetServices from Microsoft.Extensions.DependencyInjection.
Register your services as:
services.AddSingleton<IService, ServiceA>();
services.AddSingleton<IService, ServiceB>();
services.AddSingleton<IService, ServiceC>();
Then resolve with a little of Linq:
var services = serviceProvider.GetServices<IService>();
var serviceB = services.First(o => o.GetType() == typeof(ServiceB));
or
var serviceZ = services.First(o => o.Name.Equals("Z"));
(assuming that IService has a string property called "Name")
Make sure to have using Microsoft.Extensions.DependencyInjection;
Update
AspNet 2.1 source: GetServices

A factory approach is certainly viable. Another approach is to use inheritance to create individual interfaces that inherit from IService, implement the inherited interfaces in your IService implementations, and register the inherited interfaces rather than the base. Whether adding an inheritance hierarchy or factories is the "right" pattern all depends on who you speak to. I often have to use this pattern when dealing with multiple database providers in the same application that uses a generic, such as IRepository<T>, as the foundation for data access.
Example interfaces and implementations:
public interface IService
{
}
public interface IServiceA: IService
{}
public interface IServiceB: IService
{}
public interface IServiceC: IService
{}
public class ServiceA: IServiceA
{}
public class ServiceB: IServiceB
{}
public class ServiceC: IServiceC
{}
Container:
container.Register<IServiceA, ServiceA>();
container.Register<IServiceB, ServiceB>();
container.Register<IServiceC, ServiceC>();

I just simply inject an IEnumerable
ConfigureServices in Startup.cs
Assembly.GetEntryAssembly().GetTypesAssignableFrom<IService>().ForEach((t)=>
{
services.AddScoped(typeof(IService), t);
});
Services Folder
public interface IService
{
string Name { get; set; }
}
public class ServiceA : IService
{
public string Name { get { return "A"; } }
}
public class ServiceB : IService
{
public string Name { get { return "B"; } }
}
public class ServiceC : IService
{
public string Name { get { return "C"; } }
}
MyController.cs
public class MyController
{
private readonly IEnumerable<IService> _services;
public MyController(IEnumerable<IService> services)
{
_services = services;
}
public void DoSomething()
{
var service = _services.Where(s => s.Name == "A").Single();
}
...
}
Extensions.cs
public static List<Type> GetTypesAssignableFrom<T>(this Assembly assembly)
{
return assembly.GetTypesAssignableFrom(typeof(T));
}
public static List<Type> GetTypesAssignableFrom(this Assembly assembly, Type compareType)
{
List<Type> ret = new List<Type>();
foreach (var type in assembly.DefinedTypes)
{
if (compareType.IsAssignableFrom(type) && compareType != type)
{
ret.Add(type);
}
}
return ret;
}

Bit late to this party, but here is my solution:...
Startup.cs or Program.cs if Generic Handler...
services.AddTransient<IMyInterface<CustomerSavedConsumer>, CustomerSavedConsumer>();
services.AddTransient<IMyInterface<ManagerSavedConsumer>, ManagerSavedConsumer>();
IMyInterface of T Interface Setup
public interface IMyInterface<T> where T : class, IMyInterface<T>
{
Task Consume();
}
Concrete implementations of IMyInterface of T
public class CustomerSavedConsumer: IMyInterface<CustomerSavedConsumer>
{
public async Task Consume();
}
public class ManagerSavedConsumer: IMyInterface<ManagerSavedConsumer>
{
public async Task Consume();
}
Accessing the services in a controller
public class MyController
{
private readonly IMyInterface<CustomerSavedConsumer> _customerSavedConsumer;
private readonly IMyInterface<ManagerSavedConsumer> _managerSavedConsumer;
public MyController(IMyInterface<CustomerSavedConsumer> customerSavedConsumer, IMyInterface<ManagerSavedConsumer> managerSavedConsumer)
{
_customerSavedConsumer = customerSavedConsumer;
_managerSavedConsumer = managerSavedConsumer;
}
}
Hopefully if there is any issue with doing it this way, someone will kindly point out why this is the wrong way to do this.

Most of the answers here violate the single responsibility principle (a service class should not resolve dependencies itself) and/or use the service locator anti-pattern.
Another option to avoid these problems is to:
use an additional generic type parameter on the interface or a new interface implementing the non generic interface,
implement an adapter/interceptor class to add the marker type and then
use the generic type as “name”
I’ve written an article with more details: Dependency Injection in .NET: A way to work around missing named registrations

It is not supported by Microsoft.Extensions.DependencyInjection.
But you can plug-in another dependency injection mechanism, like StructureMap See it's Home page and it's GitHub Project.
It's not hard at all:
Add a dependency to StructureMap in your project.json:
"Structuremap.Microsoft.DependencyInjection" : "1.0.1",
Inject it into the ASP.NET pipeline inside ConfigureServices and register your classes (see docs)
public IServiceProvider ConfigureServices(IServiceCollection services) // returns IServiceProvider !
{
// Add framework services.
services.AddMvc();
services.AddWhatever();
//using StructureMap;
var container = new Container();
container.Configure(config =>
{
// Register stuff in container, using the StructureMap APIs...
config.For<IPet>().Add(new Cat("CatA")).Named("A");
config.For<IPet>().Add(new Cat("CatB")).Named("B");
config.For<IPet>().Use("A"); // Optionally set a default
config.Populate(services);
});
return container.GetInstance<IServiceProvider>();
}
Then, to get a named instance, you will need to request the IContainer
public class HomeController : Controller
{
public HomeController(IContainer injectedContainer)
{
var myPet = injectedContainer.GetInstance<IPet>("B");
string name = myPet.Name; // Returns "CatB"
That's it.
For the example to build, you need
public interface IPet
{
string Name { get; set; }
}
public class Cat : IPet
{
public Cat(string name)
{
Name = name;
}
public string Name {get; set; }
}

Why not use inheritance? This way we can have as many copies of the interface as we want and we can pick suitable names for each of them . And we have a benefit of type safety
public interface IReportGenerator
public interface IExcelReportGenerator : IReportGenerator
public interface IPdfReportGenerator : IReportGenerator
Concrete classes:
public class ExcelReportGenerator : IExcelReportGenerator
public class PdfReportGenerator : IPdfReportGenerator
Register:
instead of
services.AddScoped<IReportGenerator, PdfReportGenerator>();
services.AddScoped<IReportGenerator, ExcelReportGenerator>();
we have :
services.AddScoped<IPdfReportGenerator, PdfReportGenerator>();
services.AddScoped<IExcelReportGenerator, ExcelReportGenerator>();
Client:
public class ReportManager : IReportManager
{
private readonly IExcelReportGenerator excelReportGenerator;
private readonly IPdfReportGenerator pdfReportGenerator;
public ReportManager(IExcelReportGenerator excelReportGenerator,
IPdfReportGenerator pdfReportGenerator)
{
this.excelReportGenerator = excelReportGenerator;
this.pdfReportGenerator = pdfReportGenerator;
}
this approach also allows for louse coupled code, because we can move IReportGenerator to the core of the application and have child interfaces that will be declared at higher levels.

You're correct, the built in ASP.NET Core container does not have the concept of registering multiple services and then retrieving a specific one, as you suggest, a factory is the only real solution in that case.
Alternatively, you could switch to a third party container like Unity or StructureMap that does provide the solution you need (documented here: https://docs.asp.net/en/latest/fundamentals/dependency-injection.html?#replacing-the-default-services-container).

since my post above, I have moved to a Generic Factory Class
Usage
services.AddFactory<IProcessor, string>()
.Add<ProcessorA>("A")
.Add<ProcessorB>("B");
public MyClass(IFactory<IProcessor, string> processorFactory)
{
var x = "A"; //some runtime variable to select which object to create
var processor = processorFactory.Create(x);
}
Implementation
public class FactoryBuilder<I, P> where I : class
{
private readonly IServiceCollection _services;
private readonly FactoryTypes<I, P> _factoryTypes;
public FactoryBuilder(IServiceCollection services)
{
_services = services;
_factoryTypes = new FactoryTypes<I, P>();
}
public FactoryBuilder<I, P> Add<T>(P p)
where T : class, I
{
_factoryTypes.ServiceList.Add(p, typeof(T));
_services.AddSingleton(_factoryTypes);
_services.AddTransient<T>();
return this;
}
}
public class FactoryTypes<I, P> where I : class
{
public Dictionary<P, Type> ServiceList { get; set; } = new Dictionary<P, Type>();
}
public interface IFactory<I, P>
{
I Create(P p);
}
public class Factory<I, P> : IFactory<I, P> where I : class
{
private readonly IServiceProvider _serviceProvider;
private readonly FactoryTypes<I, P> _factoryTypes;
public Factory(IServiceProvider serviceProvider, FactoryTypes<I, P> factoryTypes)
{
_serviceProvider = serviceProvider;
_factoryTypes = factoryTypes;
}
public I Create(P p)
{
return (I)_serviceProvider.GetService(_factoryTypes.ServiceList[p]);
}
}
Extension
namespace Microsoft.Extensions.DependencyInjection
{
public static class DependencyExtensions
{
public static FactoryBuilder<I, P> AddFactory<I, P>(this IServiceCollection services)
where I : class
{
services.AddTransient<IFactory<I, P>, Factory<I, P>>();
return new FactoryBuilder<I, P>(services);
}
}
}

Necromancing.
I think people here are reinventing the wheel - and badly, if I may say so ...
If you want to register a component by key, just use a dictionary:
System.Collections.Generic.Dictionary<string, IConnectionFactory> dict =
new System.Collections.Generic.Dictionary<string, IConnectionFactory>(
System.StringComparer.OrdinalIgnoreCase);
dict.Add("ReadDB", new ConnectionFactory("connectionString1"));
dict.Add("WriteDB", new ConnectionFactory("connectionString2"));
dict.Add("TestDB", new ConnectionFactory("connectionString3"));
dict.Add("Analytics", new ConnectionFactory("connectionString4"));
dict.Add("LogDB", new ConnectionFactory("connectionString5"));
And then register the dictionary with the service-collection:
services.AddSingleton<System.Collections.Generic.Dictionary<string, IConnectionFactory>>(dict);
if you then are unwilling to get the dictionary and access it by key, you can hide the dictionary by adding an additional key-lookup-method to the service-collection:
(the use of delegate/closure should give a prospective maintainer a chance at understanding what's going on - the arrow-notation is a bit cryptic)
services.AddTransient<Func<string, IConnectionFactory>>(
delegate (IServiceProvider sp)
{
return
delegate (string key)
{
System.Collections.Generic.Dictionary<string, IConnectionFactory> dbs = Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService
<System.Collections.Generic.Dictionary<string, IConnectionFactory>>(sp);
if (dbs.ContainsKey(key))
return dbs[key];
throw new System.Collections.Generic.KeyNotFoundException(key); // or maybe return null, up to you
};
});
Now you can access your types with either
IConnectionFactory logDB = Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService<Func<string, IConnectionFactory>>(serviceProvider)("LogDB");
logDB.Connection
or
System.Collections.Generic.Dictionary<string, IConnectionFactory> dbs = Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService<System.Collections.Generic.Dictionary<string, IConnectionFactory>>(serviceProvider);
dbs["logDB"].Connection
As we can see, the first one is just completely superfluous, because you can also do exactly that with a dictionary, without requiring closures and AddTransient (and if you use VB, not even the braces will be different):
IConnectionFactory logDB = Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService<System.Collections.Generic.Dictionary<string, IConnectionFactory>>(serviceProvider)["logDB"];
logDB.Connection
(simpler is better - you might want to use it as extension method though)
Of course, if you don't like the dictionary, you can also outfit your interface with a property Name (or whatever), and look that up by key:
services.AddSingleton<IConnectionFactory>(new ConnectionFactory("ReadDB"));
services.AddSingleton<IConnectionFactory>(new ConnectionFactory("WriteDB"));
services.AddSingleton<IConnectionFactory>(new ConnectionFactory("TestDB"));
services.AddSingleton<IConnectionFactory>(new ConnectionFactory("Analytics"));
services.AddSingleton<IConnectionFactory>(new ConnectionFactory("LogDB"));
// https://stackoverflow.com/questions/39174989/how-to-register-multiple-implementations-of-the-same-interface-in-asp-net-core
services.AddTransient<Func<string, IConnectionFactory>>(
delegate(IServiceProvider sp)
{
return
delegate(string key)
{
System.Collections.Generic.IEnumerable<IConnectionFactory> svs =
sp.GetServices<IConnectionFactory>();
foreach (IConnectionFactory thisService in svs)
{
if (key.Equals(thisService.Name, StringComparison.OrdinalIgnoreCase))
return thisService;
}
return null;
};
});
But that requires changing your interface to accommodate the property, and looping through a lot of elements should be much slower than an associative-array lookup (dictionary).
It's nice to know that it can be done without dictionary, though.
These are just my $0.05

I've faced the same issue and want to share how I solved it and why.
As you mentioned there are two problems. The first:
In Asp.Net Core how do I register these services and resolve it at
runtime based on some key?
So what options do we have? Folks suggest two:
Use a custom factory (like _myFactory.GetServiceByKey(key))
Use another DI engine (like _unityContainer.Resolve<IService>(key))
Is the Factory pattern the only option here?
In fact both options are factories because each IoC Container is also a factory (highly configurable and complicated though). And it seems to me that other options are also variations of the Factory pattern.
So what option is better then? Here I agree with #Sock who suggested using custom factory, and that is why.
First, I always try to avoid adding new dependencies when they are not really needed. So I agree with you in this point. Moreover, using two DI frameworks is worse than creating custom factory abstraction. In the second case you have to add new package dependency (like Unity) but depending on a new factory interface is less evil here. The main idea of ASP.NET Core DI, I believe, is simplicity. It maintains a minimal set of features following KISS principle. If you need some extra feature then DIY or use a corresponding Plungin that implements desired feature (Open Closed Principle).
Secondly, often we need to inject many named dependencies for single service. In case of Unity you may have to specify names for constructor parameters (using InjectionConstructor). This registration uses reflection and some smart logic to guess arguments for the constructor. This also may lead to runtime errors if registration does not match the constructor arguments. From the other hand, when using your own factory you have full control of how to provide the constructor parameters. It's more readable and it's resolved at compile-time. KISS principle again.
The second problem:
How can _serviceProvider.GetService() inject appropriate connection
string?
First, I agree with you that depending on new things like IOptions (and therefore on package Microsoft.Extensions.Options.ConfigurationExtensions) is not a good idea. I've seen some discussing about IOptions where there were different opinions about its benifit. Again, I try to avoid adding new dependencies when they are not really needed. Is it really needed? I think no. Otherwise each implementation would have to depend on it without any clear need coming from that implementation (for me it looks like violation of ISP, where I agree with you too). This is also true about depending on the factory but in this case it can be avoided.
The ASP.NET Core DI provides a very nice overload for that purpose:
var mongoConnection = //...
var efConnection = //...
var otherConnection = //...
services.AddTransient<IMyFactory>(
s => new MyFactoryImpl(
mongoConnection, efConnection, otherConnection,
s.GetService<ISomeDependency1>(), s.GetService<ISomeDependency2>())));

Apparently, you can just inject IEnumerable of your service interface!
And then find the instance that you want using LINQ.
My example is for the AWS SNS service but you can do the same for any injected service really.
Startup
foreach (string snsRegion in Configuration["SNSRegions"].Split(',', StringSplitOptions.RemoveEmptyEntries))
{
services.AddAWSService<IAmazonSimpleNotificationService>(
string.IsNullOrEmpty(snsRegion) ? null :
new AWSOptions()
{
Region = RegionEndpoint.GetBySystemName(snsRegion)
}
);
}
services.AddSingleton<ISNSFactory, SNSFactory>();
services.Configure<SNSConfig>(Configuration);
SNSConfig
public class SNSConfig
{
public string SNSDefaultRegion { get; set; }
public string SNSSMSRegion { get; set; }
}
appsettings.json
"SNSRegions": "ap-south-1,us-west-2",
"SNSDefaultRegion": "ap-south-1",
"SNSSMSRegion": "us-west-2",
SNS Factory
public class SNSFactory : ISNSFactory
{
private readonly SNSConfig _snsConfig;
private readonly IEnumerable<IAmazonSimpleNotificationService> _snsServices;
public SNSFactory(
IOptions<SNSConfig> snsConfig,
IEnumerable<IAmazonSimpleNotificationService> snsServices
)
{
_snsConfig = snsConfig.Value;
_snsServices = snsServices;
}
public IAmazonSimpleNotificationService ForDefault()
{
return GetSNS(_snsConfig.SNSDefaultRegion);
}
public IAmazonSimpleNotificationService ForSMS()
{
return GetSNS(_snsConfig.SNSSMSRegion);
}
private IAmazonSimpleNotificationService GetSNS(string region)
{
return GetSNS(RegionEndpoint.GetBySystemName(region));
}
private IAmazonSimpleNotificationService GetSNS(RegionEndpoint region)
{
IAmazonSimpleNotificationService service = _snsServices.FirstOrDefault(sns => sns.Config.RegionEndpoint == region);
if (service == null)
{
throw new Exception($"No SNS service registered for region: {region}");
}
return service;
}
}
public interface ISNSFactory
{
IAmazonSimpleNotificationService ForDefault();
IAmazonSimpleNotificationService ForSMS();
}
Now you can get the SNS service for the region that you want in your custom service or controller
public class SmsSender : ISmsSender
{
private readonly IAmazonSimpleNotificationService _sns;
public SmsSender(ISNSFactory snsFactory)
{
_sns = snsFactory.ForSMS();
}
.......
}
public class DeviceController : Controller
{
private readonly IAmazonSimpleNotificationService _sns;
public DeviceController(ISNSFactory snsFactory)
{
_sns = snsFactory.ForDefault();
}
.........
}

Here is an example on how to create a Dependency Resolver that allows you to specify an generic argument to resolve your dependency.
var serviceProvider = new ServiceCollection()
.AddSingleton<IPerson, Larry>()
.AddSingleton<IPerson, Phil>()
.AddSingleton<IDependencyResolver<IPerson, string>, PersonDependecyResolver>()
.BuildServiceProvider();
var persons = serviceProvider.GetService<IDependencyResolver<IPerson, string>>();
Console.WriteLine(persons.GetDependency("Phil").GetName());
public interface IDependencyResolver<out TResolve, in TArg>
{
TResolve GetDependency(TArg arg);
}
public class PersonDependecyResolver : IDependencyResolver<IPerson, string>
{
private readonly IEnumerable<IPerson> people;
public PersonDependecyResolver(IEnumerable<IPerson> people)
{
this.people = people;
}
public IPerson GetDependency(string arg)
{
return arg switch
{
"Larry" => this.people.FirstOrDefault(p => p.GetType() == typeof(Larry)),
"Phil" => this.people.FirstOrDefault(p => p.GetType() == typeof(Phil)),
_ => throw new Exception("Unable to resolve dependency")
}
?? throw new Exception($"No type was found for argument {arg}");
}
}

My solution for what it's worth... considered switching to Castle Windsor as can't say I liked any of the solutions above. Sorry!!
public interface IStage<out T> : IStage { }
public interface IStage {
void DoSomething();
}
Create your various implementations
public class YourClassA : IStage<YouClassA> {
public void DoSomething()
{
...TODO
}
}
public class YourClassB : IStage<YourClassB> { .....etc. }
Registration
services.AddTransient<IStage<YourClassA>, YourClassA>()
services.AddTransient<IStage<YourClassB>, YourClassB>()
Constructor and instance usage...
public class Whatever
{
private IStage ClassA { get; }
public Whatever(IStage<YourClassA> yourClassA)
{
ClassA = yourClassA;
}
public void SomeWhateverMethod()
{
ClassA.DoSomething();
.....
}

I didn't have time to read through them all but it seemed everyone was providing solutions to problems that shouldn't exist in the first place.
If you need all of the registered IService implementations then you need them all. But DO NOT inject them all with IEnumerable and then use logic to select one based on some type of key. Problem with doing that is you need a key and the logic should not need to change if the key changes ie; different implementation of IService so typeof doesn't work any more.
The Real Problem
There is business logic here that should be in an engine service. Something like IServiceDecisionEngine is needed. The implementation of the IServiceDecisionEngine gets ONLY the needed IService implementations from DI. Like
public class ServiceDecisionEngine<SomeData>: IServiceDecisionEngine<T>
{
public ServiceDecisionEngine(IService serviceA, IService serviceB) { }
public IService ResolveService(SomeData dataNeededForLogic)
{
if (dataNeededForLogic.someValue == true)
{
return serviceA;
}
return serviceB;
}
}
Now in your DI, you can do .AddScoped<IServiceDecisionEngine<SomeData>, new ServiceDecisionEngine(new ServiceA(), new ServiceB()) and the managerService that needs an IService will get it by injecting and using IServiceDecisionEngine.

I think the solution described in the following article "Resolución dinámica de tipos en tiempo de ejecución en el contenedor de IoC de .NET Core" is simpler and does not require factories.
You could use a generic interface
public interface IService<T> where T : class {}
then register the desired types on the IoC container:
services.AddTransient<IService<ServiceA>, ServiceA>();
services.AddTransient<IService<ServiceB>, ServiceB>();
After that you must declare the dependencies as follow:
private readonly IService<ServiceA> _serviceA;
private readonly IService<ServiceB> _serviceB;
public WindowManager(IService<ServiceA> serviceA, IService<ServiceB> serviceB)
{
this._serviceA = serviceA ?? throw new ArgumentNullException(nameof(serviceA));
this._serviceB = serviceB ?? throw new ArgumentNullException(nameof(ServiceB));
}

Modular extension class solution
Very late answer, but this is the way I do it, which has some advantages over some of the other solutions to this question.
Advantages:
only 1 line of code per service implementation registration, no extra logic necessary in the registration method
the keyed services do not need to all be registered at the same time and/or place. the registrations can even be done in different projects if that is what is needed, as long as the keys are unique. this allows new implementations to be added completely modularly.
service instantiation is lazy (+ thread safe), so no unnecessary activation of all implementations when only one or a few are used.
no dependency on any external delegate or type in your code, the service is injected as a plain Func<TKey, TService> by default, but it is easy to register a custom delegate or type if you prefer
easy to choose between Transient, Singleton, or Scoped registration for the factory
use any key type you like (I do strongly suggest you just use simple types with build-in efficient equality comparison like an int, string, enum, or bool because why make life more complicated than it needs to be)
Configuration examples:
public IServiceProvider ConfigureServices(IServiceCollection services)
{
// default instantiation:
services.AddKeyedService<IService, ImplementationA, string>("A", ServiceLifetime.Scoped);
// using an implementation factory to pass a connection string to the constructor:
services.AddKeyedService<IService, ImplementationB, string>("B", x => {
var connectionString = ConfigurationManager.ConnectionStrings["mongo"].ConnectionString;
return new ImplementationB(connectionString);
}, ServiceLifetime.Scoped);
// using a custom delegate instead of Func<TKey, TService>
services.AddKeyedService<IService, ImplementationC, string, StringKeyedService>(
"C", (_, x) => new StringKeyedService(x), ServiceLifetime.Singleton);
return services.BuildServiceProvider();
}
public delegate IService StringKeyedService(string key);
Usage examples:
public ExampleClass(Func<string, IService> keyedServiceFactory, StringKeyedService<IService> keyedServiceDelegate)
{
var serviceKey = Configuration.GetValue<string>("IService.Key");
var service = keyedServiceFactory(serviceKey);
var serviceC = keyedServiceDelegate("C");
}
Implementation:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using Microsoft.Extensions.DependencyInjection;
public static class KeyedServiceExtensions
{
// Use this to register TImplementation as TService, injectable as Func<TKey, TService>.
// Uses default instance activator.
public static IServiceCollection AddKeyedService<TService, TImplementation, TKey>(this IServiceCollection services, TKey key, ServiceLifetime serviceLifetime)
where TService : class
where TImplementation : class, TService
{
services.AddTransient<TImplementation>();
var keyedServiceBuilder = services.CreateOrUpdateKeyedServiceBuilder<TKey, TService, Func<TKey, TService>>(
DefaultImplementationFactory<TKey, TService>, serviceLifetime);
keyedServiceBuilder.Add<TImplementation>(key);
return services;
}
// Use this to register TImplementation as TService, injectable as Func<TKey, TService>.
// Uses implementationFactory to create instances
public static IServiceCollection AddKeyedService<TService, TImplementation, TKey>(this IServiceCollection services, TKey key,
Func<IServiceProvider, TImplementation> implementationFactory, ServiceLifetime serviceLifetime)
where TService : class
where TImplementation : class, TService
{
services.AddTransient(implementationFactory);
var keyedServiceBuilder = services.CreateOrUpdateKeyedServiceBuilder<TKey, TService, Func<TKey, TService>>(
DefaultImplementationFactory<TKey, TService>, serviceLifetime);
keyedServiceBuilder.Add<TImplementation>(key);
return services;
}
// Use this to register TImplementation as TService, injectable as TInjection.
// Uses default instance activator.
public static IServiceCollection AddKeyedService<TService, TImplementation, TKey, TInjection>(this IServiceCollection services, TKey key,
Func<IServiceProvider, Func<TKey, TService>, TInjection> serviceFactory, ServiceLifetime serviceLifetime)
where TService : class
where TImplementation : class, TService
where TInjection : class
{
services.AddTransient<TImplementation>();
var keyedServiceBuilder = services.CreateOrUpdateKeyedServiceBuilder<TKey, TService, TInjection>(
x => serviceFactory(x, DefaultImplementationFactory<TKey, TService>(x)), serviceLifetime);
keyedServiceBuilder.Add<TImplementation>(key);
return services;
}
// Use this to register TImplementation as TService, injectable as TInjection.
// Uses implementationFactory to create instances
public static IServiceCollection AddKeyedService<TService, TImplementation, TKey, TInjection>(this IServiceCollection services, TKey key,
Func<IServiceProvider, TImplementation> implementationFactory, Func<IServiceProvider, Func<TKey, TService>, TInjection> serviceFactory, ServiceLifetime serviceLifetime)
where TService : class
where TImplementation : class, TService
where TInjection : class
{
services.AddTransient(implementationFactory);
var keyedServiceBuilder = services.CreateOrUpdateKeyedServiceBuilder<TKey, TService, TInjection>(
x => serviceFactory(x, DefaultImplementationFactory<TKey, TService>(x)), serviceLifetime);
keyedServiceBuilder.Add<TImplementation>(key);
return services;
}
private static KeyedServiceBuilder<TKey, TService> CreateOrUpdateKeyedServiceBuilder<TKey, TService, TInjection>(this IServiceCollection services,
Func<IServiceProvider, TInjection> serviceFactory, ServiceLifetime serviceLifetime)
where TService : class
where TInjection : class
{
var builderServiceDescription = services.SingleOrDefault(x => x.ServiceType == typeof(KeyedServiceBuilder<TKey, TService>));
KeyedServiceBuilder<TKey, TService> keyedServiceBuilder;
if (builderServiceDescription is null)
{
keyedServiceBuilder = new KeyedServiceBuilder<TKey, TService>();
services.AddSingleton(keyedServiceBuilder);
switch (serviceLifetime)
{
case ServiceLifetime.Singleton:
services.AddSingleton(serviceFactory);
break;
case ServiceLifetime.Scoped:
services.AddScoped(serviceFactory);
break;
case ServiceLifetime.Transient:
services.AddTransient(serviceFactory);
break;
default:
throw new ArgumentOutOfRangeException(nameof(serviceLifetime), serviceLifetime, "Invalid value for " + nameof(serviceLifetime));
}
}
else
{
CheckLifetime<KeyedServiceBuilder<TKey, TService>>(builderServiceDescription.Lifetime, ServiceLifetime.Singleton);
var factoryServiceDescriptor = services.SingleOrDefault(x => x.ServiceType == typeof(TInjection));
CheckLifetime<TInjection>(factoryServiceDescriptor.Lifetime, serviceLifetime);
keyedServiceBuilder = (KeyedServiceBuilder<TKey, TService>)builderServiceDescription.ImplementationInstance;
}
return keyedServiceBuilder;
static void CheckLifetime<T>(ServiceLifetime actual, ServiceLifetime expected)
{
if (actual != expected)
throw new ApplicationException($"{typeof(T).FullName} is already registered with a different ServiceLifetime. Expected: '{expected}', Actual: '{actual}'");
}
}
private static Func<TKey, TService> DefaultImplementationFactory<TKey, TService>(IServiceProvider x) where TService : class
=> x.GetRequiredService<KeyedServiceBuilder<TKey, TService>>().Build(x);
private sealed class KeyedServiceBuilder<TKey, TService>
{
private readonly Dictionary<TKey, Type> _serviceImplementationTypes = new Dictionary<TKey, Type>();
internal void Add<TImplementation>(TKey key) where TImplementation : class, TService
{
if (_serviceImplementationTypes.TryGetValue(key, out var type) && type == typeof(TImplementation))
return; //this type is already registered under this key
_serviceImplementationTypes[key] = typeof(TImplementation);
}
internal Func<TKey, TService> Build(IServiceProvider serviceProvider)
{
var serviceTypeDictionary = _serviceImplementationTypes.Values.Distinct()
.ToDictionary(
type => type,
type => new Lazy<TService>(
() => (TService)serviceProvider.GetRequiredService(type),
LazyThreadSafetyMode.ExecutionAndPublication
)
);
var serviceDictionary = _serviceImplementationTypes
.ToDictionary(kvp => kvp.Key, kvp => serviceTypeDictionary[kvp.Value]);
return key => serviceDictionary[key].Value;
}
}
}
it's also possible to make a fluid interface on top of this, let me know if there is interest in that.
Example fluid usage:
var keyedService = services.KeyedSingleton<IService, ServiceKey>()
.As<ICustomKeyedService<TKey, IService>>((_, x) => new CustomKeyedServiceInterface<ServiceKey, IService>(x));
keyedService.Key(ServiceKey.A).Add<ServiceA>();
keyedService.Key(ServiceKey.B).Add(x => {
x.GetService<ILogger>.LogDebug("Instantiating ServiceB");
return new ServiceB();
});

I had the same problem and I solved using <T>
My interface:
public interface IProvider<T>
{
Task<string> GetTotalSearchResults(string searchValue);
}
My services configuration:
var host = Host.CreateDefaultBuilder()
.ConfigureServices((_, services) =>
{
services.AddSingleton(googleSettings);
services.AddSingleton(bingSettings);
services.AddSingleton<IProvider<BingProvider>, BingProvider>();
services.AddSingleton<IProvider<GoogleProvider>, GoogleProvider>();
services.AddSingleton<ISearchManager, SearchManager>();
});
And the you can use it in your class:
public class SearchManager : ISearchManager
{
private readonly IProvider<BingProvider> _bing;
private readonly IProvider<GoogleProvider> _google;
public SearchManager(IProvider<BingProvider> bing, IProvider<GoogleProvider> google)
{
_bing = bing;
_google = google;
}

The best documentation/tutorial I found for multiple implementation are from this source:
.NET Core Dependency Injection - One Interface, Multiple Implementations, (Authored by Akshay Patel)
Example mentioned in the tutorial follows the Controller/Service/Repository convention, with Func implementation in the ConfigurationService() from Startup.cs to instantiate the proper/needed interface implementation. Tutorial was the best recipe I found to clarify this issue.
Below, a crude copy/paste from the article mentioned above: (example deals with 3 different implementations of a shopping cart interface, one method with a cache solution, another with API and other implementation with DB.)
Interface to be multiple implemented....
namespace MultipleImplementation
{
public interface IShoppingCart
{
object GetCart();
}
}
implementation A
namespace MultipleImplementation
{
public class ShoppingCartCache : IShoppingCart
{
public object GetCart()
{
return "Cart loaded from cache.";
}
}
}
Implementation B
namespace MultipleImplementation
{
public class ShoppingCartDB : IShoppingCart
{
public object GetCart()
{
return "Cart loaded from DB";
}
}
}
Implementation C
namespace MultipleImplementation
{
public class ShoppingCartAPI : IShoppingCart
{
public object GetCart()
{
return "Cart loaded through API.";
}
}
}
An Interface Declaration in Repository to select rather A,B,C will be used....
namespace MultipleImplementation
{
public interface IShoppingCartRepository
{
object GetCart();
}
}
enum to select which implementation will be used...
namespace MultipleImplementation
{
public class Constants
{
}
public enum CartSource
{
Cache=1,
DB=2,
API=3
}
}
The implementation of the declared repository interface (who will select which implementation... )
using System;
namespace MultipleImplementation
{
public class ShoppingCartRepository : IShoppingCartRepository
{
private readonly Func<string, IShoppingCart> shoppingCart;
public ShoppingCartRepository(Func<string, IShoppingCart> shoppingCart)
{
this.shoppingCart = shoppingCart;
}
public object GetCart()
{
return shoppingCart(CartSource.DB.ToString()).GetCart();
}
}
}
Finally, packing all together in the startup.cs file, in ConfigureService method
public void ConfigureServices(IServiceCollection services)
{
services.AddScoped<IShoppingCartRepository, ShoppingCartRepository>();
services.AddSingleton<ShoppingCartCache>();
services.AddSingleton<ShoppingCartDB>();
services.AddSingleton<ShoppingCartAPI>();
services.AddTransient<Func<string, IShoppingCart>>(serviceProvider => key =>
{
switch (key)
{
case "API":
return serviceProvider.GetService<ShoppingCartAPI>();
case "DB":
return serviceProvider.GetService<ShoppingCartDB>();
default:
return serviceProvider.GetService<ShoppingCartCache>();
}
});
services.AddMvc();
}
There, I reinforce, that a 6 min read will clear the mind to help you solve multiple implementations into one interface. Good luck!

While it seems #Miguel A. Arilla has pointed it out clearly and I voted up for him, I created on top of his useful solution another solution which looks neat but requires a lot more work.
It definitely depends on the above solution. So basically I created something similar to Func<string, IService>> and I called it IServiceAccessor as an interface and then I had to add a some more extensions to the IServiceCollection as such:
public static IServiceCollection AddSingleton<TService, TImplementation, TServiceAccessor>(
this IServiceCollection services,
string instanceName
)
where TService : class
where TImplementation : class, TService
where TServiceAccessor : class, IServiceAccessor<TService>
{
services.AddSingleton<TService, TImplementation>();
services.AddSingleton<TServiceAccessor>();
var provider = services.BuildServiceProvider();
var implementationInstance = provider.GetServices<TService>().Last();
var accessor = provider.GetServices<TServiceAccessor>().First();
var serviceDescriptors = services.Where(d => d.ServiceType == typeof(TServiceAccessor));
while (serviceDescriptors.Any())
{
services.Remove(serviceDescriptors.First());
}
accessor.SetService(implementationInstance, instanceName);
services.AddSingleton<TServiceAccessor>(prvd => accessor);
return services;
}
The service Accessor looks like:
public interface IServiceAccessor<TService>
{
void Register(TService service,string name);
TService Resolve(string name);
}
The end result,you will be able to register services with names or named instances like we used to do with other containers..for instance:
services.AddSingleton<IEncryptionService, SymmetricEncryptionService, EncyptionServiceAccessor>("Symmetric");
services.AddSingleton<IEncryptionService, AsymmetricEncryptionService, EncyptionServiceAccessor>("Asymmetric");
That is enough for now, but to make your work complete, it is better to add more extension methods as you can to cover all types of registrations following the same approach.
There was another post on stackoverflow, but I can not find it, where the poster has explained in details why this feature is not supported and how to work around it, basically similar to what #Miguel stated. It was nice post even though I do not agree with each point because I think there are situation where you really need named instances. I will post that link here once I find it again.
As a matter of fact, you do not need to pass that Selector or Accessor:
I am using the following code in my project and it worked well so far.
/// <summary>
/// Adds the singleton.
/// </summary>
/// <typeparam name="TService">The type of the t service.</typeparam>
/// <typeparam name="TImplementation">The type of the t implementation.</typeparam>
/// <param name="services">The services.</param>
/// <param name="instanceName">Name of the instance.</param>
/// <returns>IServiceCollection.</returns>
public static IServiceCollection AddSingleton<TService, TImplementation>(
this IServiceCollection services,
string instanceName
)
where TService : class
where TImplementation : class, TService
{
var provider = services.BuildServiceProvider();
var implementationInstance = provider.GetServices<TService>().LastOrDefault();
if (implementationInstance.IsNull())
{
services.AddSingleton<TService, TImplementation>();
provider = services.BuildServiceProvider();
implementationInstance = provider.GetServices<TService>().Single();
}
return services.RegisterInternal(instanceName, provider, implementationInstance);
}
private static IServiceCollection RegisterInternal<TService>(this IServiceCollection services,
string instanceName, ServiceProvider provider, TService implementationInstance)
where TService : class
{
var accessor = provider.GetServices<IServiceAccessor<TService>>().LastOrDefault();
if (accessor.IsNull())
{
services.AddSingleton<ServiceAccessor<TService>>();
provider = services.BuildServiceProvider();
accessor = provider.GetServices<ServiceAccessor<TService>>().Single();
}
else
{
var serviceDescriptors = services.Where(d => d.ServiceType == typeof(IServiceAccessor<TService>));
while (serviceDescriptors.Any())
{
services.Remove(serviceDescriptors.First());
}
}
accessor.Register(implementationInstance, instanceName);
services.AddSingleton<TService>(prvd => implementationInstance);
services.AddSingleton<IServiceAccessor<TService>>(prvd => accessor);
return services;
}
//
// Summary:
// Adds a singleton service of the type specified in TService with an instance specified
// in implementationInstance to the specified Microsoft.Extensions.DependencyInjection.IServiceCollection.
//
// Parameters:
// services:
// The Microsoft.Extensions.DependencyInjection.IServiceCollection to add the service
// to.
// implementationInstance:
// The instance of the service.
// instanceName:
// The name of the instance.
//
// Returns:
// A reference to this instance after the operation has completed.
public static IServiceCollection AddSingleton<TService>(
this IServiceCollection services,
TService implementationInstance,
string instanceName) where TService : class
{
var provider = services.BuildServiceProvider();
return RegisterInternal(services, instanceName, provider, implementationInstance);
}
/// <summary>
/// Registers an interface for a class
/// </summary>
/// <typeparam name="TInterface">The type of the t interface.</typeparam>
/// <param name="services">The services.</param>
/// <returns>IServiceCollection.</returns>
public static IServiceCollection As<TInterface>(this IServiceCollection services)
where TInterface : class
{
var descriptor = services.Where(d => d.ServiceType.GetInterface(typeof(TInterface).Name) != null).FirstOrDefault();
if (descriptor.IsNotNull())
{
var provider = services.BuildServiceProvider();
var implementationInstance = (TInterface)provider?.GetServices(descriptor?.ServiceType)?.Last();
services?.AddSingleton(implementationInstance);
}
return services;
}

I have created a library for this that implements some nice features.
Code can be found on GitHub: https://github.com/dazinator/Dazinator.Extensions.DependencyInjection
NuGet: https://www.nuget.org/packages/Dazinator.Extensions.DependencyInjection/
Usage is straightforward:
Add the Dazinator.Extensions.DependencyInjection nuget package to your project.
Add your Named Service registrations.
var services = new ServiceCollection();
services.AddNamed<AnimalService>(names =>
{
names.AddSingleton("A"); // will resolve to a singleton instance of AnimalService
names.AddSingleton<BearService>("B"); // will resolve to a singleton instance of BearService (which derives from AnimalService)
names.AddSingleton("C", new BearService()); will resolve to singleton instance provided yourself.
names.AddSingleton("D", new DisposableTigerService(), registrationOwnsInstance = true); // will resolve to singleton instance provided yourself, but will be disposed for you (if it implements IDisposable) when this registry is disposed (also a singleton).
names.AddTransient("E"); // new AnimalService() every time..
names.AddTransient<LionService>("F"); // new LionService() every time..
names.AddScoped("G"); // scoped AnimalService
names.AddScoped<DisposableTigerService>("H"); scoped DisposableTigerService and as it implements IDisposable, will be disposed of when scope is disposed of.
});
In the example above, notice that for each named registration, you are also specifying the lifetime or Singleton, Scoped, or Transient.
You can resolve services in one of two ways, depending on if you are comfortable with having your services take a dependency on this package of not:
public MyController(Func<string, AnimalService> namedServices)
{
AnimalService serviceA = namedServices("A");
AnimalService serviceB = namedServices("B"); // BearService derives from AnimalService
}
or
public MyController(NamedServiceResolver<AnimalService> namedServices)
{
AnimalService serviceA = namedServices["A"];
AnimalService serviceB = namedServices["B"]; // instance of BearService returned derives from AnimalService
}
I have specifically designed this library to work well with Microsoft.Extensions.DependencyInjection - for example:
When you register named services, any types that you register can have constructors with parameters - they will be satisfied via DI, in the same way that AddTransient<>, AddScoped<> and AddSingleton<> methods work ordinarily.
For transient and scoped named services, the registry builds an ObjectFactory so that it can activate new instances of the type very quickly when needed. This is much faster than other approaches and is in line with how Microsoft.Extensions.DependencyInjection does things.

I created my own extension over IServiceCollection used WithName extension:
public static IServiceCollection AddScopedWithName<TService, TImplementation>(this IServiceCollection services, string serviceName)
where TService : class
where TImplementation : class, TService
{
Type serviceType = typeof(TService);
Type implementationServiceType = typeof(TImplementation);
ServiceCollectionTypeMapper.Instance.AddDefinition(serviceType.Name, serviceName, implementationServiceType.AssemblyQualifiedName);
services.AddScoped<TImplementation>();
return services;
}
ServiceCollectionTypeMapper is a singleton instance that maps IService > NameOfService > Implementation where an interface could have many implementations with different names, this allows to register types than we can resolve when wee need and is a different approach than resolve multiple services to select what we want.
/// <summary>
/// Allows to set the service register mapping.
/// </summary>
public class ServiceCollectionTypeMapper
{
private ServiceCollectionTypeMapper()
{
this.ServiceRegister = new Dictionary<string, Dictionary<string, string>>();
}
/// <summary>
/// Gets the instance of mapper.
/// </summary>
public static ServiceCollectionTypeMapper Instance { get; } = new ServiceCollectionTypeMapper();
private Dictionary<string, Dictionary<string, string>> ServiceRegister { get; set; }
/// <summary>
/// Adds new service definition.
/// </summary>
/// <param name="typeName">The name of the TService.</param>
/// <param name="serviceName">The TImplementation name.</param>
/// <param name="namespaceFullName">The TImplementation AssemblyQualifiedName.</param>
public void AddDefinition(string typeName, string serviceName, string namespaceFullName)
{
if (this.ServiceRegister.TryGetValue(typeName, out Dictionary<string, string> services))
{
if (services.TryGetValue(serviceName, out _))
{
throw new InvalidOperationException($"Exists an implementation with the same name [{serviceName}] to the type [{typeName}].");
}
else
{
services.Add(serviceName, namespaceFullName);
}
}
else
{
Dictionary<string, string> serviceCollection = new Dictionary<string, string>
{
{ serviceName, namespaceFullName },
};
this.ServiceRegister.Add(typeName, serviceCollection);
}
}
/// <summary>
/// Get AssemblyQualifiedName of implementation.
/// </summary>
/// <typeparam name="TService">The type of the service implementation.</typeparam>
/// <param name="serviceName">The name of the service.</param>
/// <returns>The AssemblyQualifiedName of the inplementation service.</returns>
public string GetService<TService>(string serviceName)
{
Type serviceType = typeof(TService);
if (this.ServiceRegister.TryGetValue(serviceType.Name, out Dictionary<string, string> services))
{
if (services.TryGetValue(serviceName, out string serviceImplementation))
{
return serviceImplementation;
}
else
{
return null;
}
}
else
{
return null;
}
}
To register a new service:
services.AddScopedWithName<IService, MyService>("Name");
To resolve service we need an extension over IServiceProvider like this.
/// <summary>
/// Gets the implementation of service by name.
/// </summary>
/// <typeparam name="T">The type of service.</typeparam>
/// <param name="serviceProvider">The service provider.</param>
/// <param name="serviceName">The service name.</param>
/// <returns>The implementation of service.</returns>
public static T GetService<T>(this IServiceProvider serviceProvider, string serviceName)
{
string fullnameImplementation = ServiceCollectionTypeMapper.Instance.GetService<T>(serviceName);
if (fullnameImplementation == null)
{
throw new InvalidOperationException($"Unable to resolve service of type [{typeof(T)}] with name [{serviceName}]");
}
else
{
return (T)serviceProvider.GetService(Type.GetType(fullnameImplementation));
}
}
When resolve:
serviceProvider.GetService<IWithdrawalHandler>(serviceName);
Remember that serviceProvider can be injected within a constructor in our application as IServiceProvider.
I hope this helps.

I know this post is a couple years old, but I keep running into this and I'm not happy with the service locator pattern.
Also, I know the OP is looking for an implementation which allows you to choose a concrete implementation based on a string. I also realize that the OP is specifically asking for an implementation of an identical interface. The solution I'm about to describe relies on adding a generic type parameter to your interface. The problem is that you don't have any real use for the type parameter other than service collection binding. I'll try to describe a situation which might require something like this.
Imagine configuration for such a scenario in appsettings.json which might look something like this (this is just for demonstration, your configuration can come from wherever you want as long as you have the correction configuration provider):
{
"sqlDataSource": {
"connectionString": "Data Source=localhost; Initial catalog=Foo; Connection Timeout=5; Encrypt=True;",
"username": "foo",
"password": "this normally comes from a secure source, but putting here for demonstration purposes"
},
"mongoDataSource": {
"hostName": "uw1-mngo01-cl08.company.net",
"port": 27026,
"collection": "foo"
}
}
You really need a type that represents each of your configuration options:
public class SqlDataSource
{
public string ConnectionString { get;set; }
public string Username { get;set; }
public string Password { get;set; }
}
public class MongoDataSource
{
public string HostName { get;set; }
public string Port { get;set; }
public string Collection { get;set; }
}
Now, I know that it might seem a little contrived to have two implementations of the same interface, but it I've definitely seen it in more than one case. The ones I usually come across are:
When migrating from one data store to another, it's useful to be able to implement the same logical operations using the same interfaces so that you don't need to change the calling code. This also allows you to add configuration which swaps between different implementations at runtime (which can be useful for rollback).
When using the decorator pattern. The reason you might use that pattern is that you want to add functionality without changing the interface and fall back to the existing functionality in certain cases (I've used it when adding caching to repository classes because I want circuit breaker-like logic around connections to the cache that fall back to the base repository -- this gives me optimal behavior when the cache is available, but behavior that still functions when it's not).
Anyway, you can reference them by adding a type parameter to your service interface so that you can implement the different implementations:
public interface IService<T> {
void DoServiceOperation();
}
public class MongoService : IService<MongoDataSource> {
private readonly MongoDataSource _options;
public FooService(IOptionsMonitor<MongoDataSource> serviceOptions){
_options = serviceOptions.CurrentValue
}
void DoServiceOperation(){
//do something with your mongo data source options (connect to database)
throw new NotImplementedException();
}
}
public class SqlService : IService<SqlDataSource> {
private readonly SqlDataSource_options;
public SqlService (IOptionsMonitor<SqlDataSource> serviceOptions){
_options = serviceOptions.CurrentValue
}
void DoServiceOperation(){
//do something with your sql data source options (connect to database)
throw new NotImplementedException();
}
}
In startup, you'd register these with the following code:
services.Configure<SqlDataSource>(configurationSection.GetSection("sqlDataSource"));
services.Configure<MongoDataSource>(configurationSection.GetSection("mongoDataSource"));
services.AddTransient<IService<SqlDataSource>, SqlService>();
services.AddTransient<IService<MongoDataSource>, MongoService>();
Finally in the class which relies on the Service with a different connection, you just take a dependency on the service you need and the DI framework will take care of the rest:
[Route("api/v1)]
[ApiController]
public class ControllerWhichNeedsMongoService {
private readonly IService<MongoDataSource> _mongoService;
private readonly IService<SqlDataSource> _sqlService ;
public class ControllerWhichNeedsMongoService(
IService<MongoDataSource> mongoService,
IService<SqlDataSource> sqlService
)
{
_mongoService = mongoService;
_sqlService = sqlService;
}
[HttpGet]
[Route("demo")]
public async Task GetStuff()
{
if(useMongo)
{
await _mongoService.DoServiceOperation();
}
await _sqlService.DoServiceOperation();
}
}
These implementations can even take a dependency on each other. The other big benefit is that you get compile-time binding so any refactoring tools will work correctly.
Hope this helps someone in the future.

Any technical way using IEnumerable<Interface> effectively defeats the whole purpose of DI since you need to select which implementation you need to resolve to and might be pointing to bad design.
The workaround for this issue that worked for me was to separate usage and create separate interfaces like so
public interface ICRUDService<T> where T : class
{
void CreateAndSetId(T item);
void Delete(int id);
ActionResult<List<T>> GetAll();
ActionResult<T> GetById(int id);
void Update(int id, T item);
}
Then the individual interfaces
public interface ITodoService : ICRUDService<Todo> {}
public interface IValuesService : ICRUDService<Values> {}
And their implementations
public class TodoService : ITodoService { ... }
public class ValuesService : IValuesService { ... }
Startup.ConfigureServices
services.AddScoped<ITodoService, TodoService>();
services.AddScoped<IValuesService, ValuesService>();
Usage
public class UsageClass {
public UsageClass(ITodoService todoService, IValuesService valuesService) {}
}
If you are still interested in resolving multiple implementations, THIS is the Microsoft recommendation. Just linking it here since this isn't what I recommend.

Extending the solution of #rnrneverdies. Instead of ToString(), following options can also be used- 1) With common property implementation, 2) A service of services suggested by #Craig Brunetti.
public interface IService { }
public class ServiceA : IService
{
public override string ToString()
{
return "A";
}
}
public class ServiceB : IService
{
public override string ToString()
{
return "B";
}
}
/// <summary>
/// extension method that compares with ToString value of an object and returns an object if found
/// </summary>
public static class ServiceProviderServiceExtensions
{
public static T GetService<T>(this IServiceProvider provider, string identifier)
{
var services = provider.GetServices<T>();
var service = services.FirstOrDefault(o => o.ToString() == identifier);
return service;
}
}
public void ConfigureServices(IServiceCollection services)
{
//Initials configurations....
services.AddSingleton<IService, ServiceA>();
services.AddSingleton<IService, ServiceB>();
services.AddSingleton<IService, ServiceC>();
var sp = services.BuildServiceProvider();
var a = sp.GetService<IService>("A"); //returns instance of ServiceA
var b = sp.GetService<IService>("B"); //returns instance of ServiceB
//Remaining configurations....
}

After reading the answers here and articles elsewhere I was able to get it working without strings. When you have multiple implementations of the same interface the DI will add these to a collection, so it's then possible to retrieve the version you want from the collection using typeof.
// In Startup.cs
public void ConfigureServices(IServiceCollection services)
{
services.AddScoped(IService, ServiceA);
services.AddScoped(IService, ServiceB);
services.AddScoped(IService, ServiceC);
}
// Any class that uses the service(s)
public class Consumer
{
private readonly IEnumerable<IService> _myServices;
public Consumer(IEnumerable<IService> myServices)
{
_myServices = myServices;
}
public UseServiceA()
{
var serviceA = _myServices.FirstOrDefault(t => t.GetType() == typeof(ServiceA));
serviceA.DoTheThing();
}
public UseServiceB()
{
var serviceB = _myServices.FirstOrDefault(t => t.GetType() == typeof(ServiceB));
serviceB.DoTheThing();
}
public UseServiceC()
{
var serviceC = _myServices.FirstOrDefault(t => t.GetType() == typeof(ServiceC));
serviceC.DoTheThing();
}
}

Okay, here a clean and readable answer by using a dictionary
Create a enum with your database Key Name
public enum Database
{
Red,
Blue
}
In Startup.cs, create a dictionary of function that open a new SqlConnection, then inject the dependency dictionary as Singleton
Dictionary<Database, Func<IDbConnection>> connectionFactory = new()
{
{ Database.Red, () => new SqlConnection(Configuration.GetConnectionString("RedDatabase")) },
{ Database.Blue, () => new SqlConnection(Configuration.GetConnectionString("BlueDatabase")) }
};
services.AddSingleton(connectionFactory);
After you can get the instance od the dependency on object constructor like so:
public class ObjectQueries
{
private readonly IDbConnection _redConnection;
private readonly IDbConnection _blueConnection;
public ObjectQueries(Dictionary<Database, Func<IDbConnection>> connectionFactory)
{
_redConnection = connectionFactory[Database.Red]();
_blueConnection = connectionFactory[Database.Blue]();
}
}
Thanks #Stefan Steiger for the Idea ;)

I really am super late to the party, but this is so super easy to solve without any kind of factory pattern or complicated jiggery...
I faced this problem and came up with a super simple approach. All you need is a container to put your object in and then register the container.
So assuming you have this (which is completely reusable):
public class DependencyRegistration<TScope, TDependency>
{
public TDependency Dependency { get; }
public DependencyRegistration(TDependency dependency)
{
Dependency = dependency;
}
}
Then you can register your services 'dependently':
.AddSingleton(serviceProvider =>
{
return new DependencyRegistration<IWidgetRepository, string>("the connection string");
})
.AddSingleton(serviceProvider =>
{
return new DependencyRegistration<IRestRequest, string>("the URL");
})
And because the TScope effectively takes the place of the named registration you just consume them thus:
[Inject]
public DependencyRegistration<IWidgetRepository, string> widgetConnectionStringRegistration { get; set; }
private string widgetConnectionString => widgetConnectionStringRegistration.Dependency;
So no violations of principles at all, and all you need is a unique type to use for the scope - use anything that makes sense or roll your own definition if you want to be semantically precise:
public class TokenContext
{
public interface IAdministrationToken { }
public interface IUserToken { }
}
And so:
.AddSingleton(serviceProvider =>
{
return new DependencyRegistration<TokenContext.IUserToken, string>("the user token");
})
.AddSingleton(serviceProvider =>
{
return new DependencyRegistration<TokenContext.IAdministrationToken, string>("the admin token");
})
And instead of the 'GetRequiredService' extension method, I created a 'GetRequiredRegisteredService' version to more easily resolve interdependent dependencies:
.AddSingleton(serviceProvider =>
{
var myURL = serviceProvider.GetRequiredRegisteredService<IRestRequest, string>();
return new new RestRequest(myURL);
})
Once you see how deceptively easy the fix is, it's then super easy to use.
Oh, and even super easier to install:
Install-Package CodeRed.Extensions.DependencyInjection.DependencyRegister
Enjoy!
Paul

While the out of the box implementation doesn't offer it, here's a sample project that allows you to register named instances, and then inject INamedServiceFactory into your code and pull out instances by name. Unlike other facory solutions here, it will allow you to register multiple instances of same implementation but configured differently
https://github.com/macsux/DotNetDINamedInstances

Inject object using MEF with information about destination object

I am looking for a way to inject a logging object which wraps a log4net logger into various objects using MEF. The problem I currently have is that the logging object requires the type of the object it is a member of. I could get round this problem by setting the type property on the logging object within the constructor of the containing object, however this leaves the onus of setting the type on the developer, and there is no compile time restriction I can think of to enforce this.
I there a way to specify that when the logging object is generated by MEF and injected, its constructor parameter is set to the type of the target class of the injection?
My logger implements an interface
public interface ILogger
{
Type Type { get; }
}
An Example of the concrete implementation of this is
[Export(typeof(Ilogger))]
public class SimpleLogger : ILogger
{
public SimpleLogger(Type typeOfObjectToLogFor)
{
this.Type = typeOfObjectToLogFor
}
public Type Type { get; }
public void Info(string message)
{
//log the messsage including the type information
}
}
and it is currently consumed not using MEF as:
public class ExampleObject
{
private readonly ILogger logger = new SimpleLogger(typeof(ExampleObject));
public ExampleObject(){}
public void MethodThatLogs()
{
logger.Info("MethodThatLogs called");
}
}
and what I would like to do is inject it using constructor injection:
public class ExampleObject
{
private readonly ILogger logger;
[ImportingConstructor]
public ExampleObject(Ilogger injectedLogger)
{
logger = injectedLogger;
}
public void MethodThatLogs()
{
logger?.Info("MethodThatLogs called");
}
}
I could do all this with lazy evaluated reflection, but it feels like something that should be possible from a decent DI container, and hopefully that means that MEF will support it, cam anyone help?

By default, specifying an [Export] attribute, you're setting the PartCreationPolicy to Shared, which means that the container will create a singleton for your export - your logger.
But I suggest you to export not a class but a factory method that will accept one argument and create the logger for you.
class LoggerFactory
{
[Export("GetLogger")]
public ILogger GetLogger(Type type)
{
return new SimpleLogger(type);
}
}
class ExampleObject
{
private readonly ILogger logger;
[ImportingConstructor]
public ExampleObject([Import(ContractName = "GetLogger", AllowDefault = true)]Func<Type, ILogger> loggerCreator)
{
logger = loggerCreator?.Invoke(this.GetType());
}
}

Way to add an Extension method?

Say I have a class called LogHelper and it has a static method called GetLogger(string name).
Is there a way to add a static extension method that would just be GetLogger()?
I know that normally extension methods are static where they are declared, but is there a way to appear static on the class they are "Helping"?

You could use an extension method:
public static class StringExtensions
{
public static ILogger GetLogger(this string value)
{
...
}
}
And then you could use like this:
string foo = "bar";
ILogger logger = foo.GetLogger();

You can also use
public static ILogger GetLogger(string value)
{
...
}
public static ILogger GetLogger()
{
return GetLogger("Default Value");
}
This way when you called GetLogger() it would call GetLogger(string value) but with your default value.

It is not possible to extend static classes with extension methods, unfortunately...
(I'm assuming your LogHelper is static? At least that's what I understood so far.)

I'm not sure how efficient this is, but it should work. log4net has an overload that accepts a Type, and you often want to name the logger after the fully qualified type name.
void Main()
{
var foo = new Bar.Foo();
ILog logger = foo.GetLogger();
}
public static class LogHelper
{
public static ILog GetLogger(this object o)
{
return LogManager.GetLogger(o.GetType());
}
}
namespace Bar
{
public class Foo {}
}
// Faking log4net
public class ILog {}
public class LogManager
{
public static ILog GetLogger(Type type)
{
Console.WriteLine("Logger: " + type.ToString());
return null;
}
}
The above prints "Logger: Bar.Foo".
P.S. Use "this.GetLogger()" if you want a logger for the current class (assuming non-static method).

Since what you want is impossible, why not fake it?
After all, it doesn't really matter if your parameterless GetLogger() method is on LogHelper. Just write your own static class, called MyLogHelper or something, and define the method there. The code would look like this:
public static class MyLogHelper
{
public static ILog GetLogger()
{
return LogHelper.GetLogger("MyHardcodedValue");
}
}

I see only one solution is to use optional arguments, but unfortunately they appear only in C# 4.0:
public static class LogHelper
{
public static ILogger GetLogger(string name = "DefaultLoggerName")
{
...
}
}