.NET Core DI, ways of passing parameters to constructor

Question

The expression parameter (x in this case) of the factory delegate is an IServiceProvider.

Use that to resolve the dependencies:

_serviceCollection.AddSingleton<IService>(x => 
    new Service(x.GetRequiredService<IOtherService>(),
                x.GetRequiredService<IAnotherOne>(), 
                ""));

The factory delegate is a delayed invocation. Whenever the type is to be resolved, it will pass the completed provider as the delegate parameter.

The recommended way to achieve this is to use the Options pattern - note that this applies to any .NET Core/5 application, not just ASP.NET Core. But there are use cases where it's impractical (e.g. when parameters are only known at runtime, not at startup/compile-time) or you need to dynamically replace a dependency.

It's very useful when you need to replace a single dependency (be it a string, integer or another type of dependency) or when using a 3rd-party library which accepts only string/integer parameters and you require runtime parameters.

You could try CreateInstance<T>(IServiceProvider, Object[]) as a shortcut rather than resolving every single dependency manually:

_serviceCollection.AddSingleton<IService>(x => 
    ActivatorUtilities.CreateInstance<Service>(x, "");
);

The parameters to pass to your service's constructor (the object[] parameter to CreateInstance<T>/CreateInstance) allows you to specify parameters that should be injected directly, as opposed to resolved from the service provider. They are applied from left to right as they appear (i.e. first string will be replaced with the first string-typed parameter of the type to be instantiated).

ActivatorUtilities.CreateInstance<Service> is used in many places to resolve service and replace one of the default registrations for this single activation.

For example, if you have a class named MyService, and it has IOtherService, ILogger<MyService> as dependencies and you want to resolve the service but replace the default service of IOtherService (say it's OtherServiceA) with OtherServiceB, you could do something like:

myService = ActivatorUtilities.CreateInstance<Service>(serviceProvider,
    new OtherServiceB());

Then the first parameter of IOtherService will get OtherServiceB injected, rather than OtherServiceA - but the remaining parameters will come from the service provider.

This is helpful when you have many dependencies and want just to treat a single one specially (i.e. replace a database-specific provider with a value configured during the request or for a specific user, something you only know at runtime and/or during a request - not when the application is built/started).

If performance is a concern, you can use ActivatorUtilities.CreateFactory(Type, Type[]) to create a factory method instead. GitHub reference and benchmark.

This is useful when the type is resolved very frequently (such as in SignalR and other high request scenarios). Basically, you'd create an ObjectFactory via

var myServiceFactory = ActivatorUtilities.CreateFactory(typeof(MyService), new object[] { typeof(IOtherService), });

then cache it (as a variable etc.) and invoke it where needed:

MyService myService = myServiceFactory(serviceProvider, myServiceOrParameterTypeToReplace);

This all works perfectly with primitive types too - here's an example I tested with:

class Program
{
    static void Main(string[] args)
    {
        var services = new ServiceCollection();
        services.AddTransient<HelloWorldService>();
        services.AddTransient(p => p.ResolveWith<DemoService>("Tseng", "Stackoverflow"));

        var provider = services.BuildServiceProvider();

        var demoService = provider.GetRequiredService<DemoService>();

        Console.WriteLine($"Output: {demoService.HelloWorld()}");
        Console.ReadKey();
    }
}

public class DemoService
{
    private readonly HelloWorldService helloWorldService;
    private readonly string firstname;
    private readonly string lastname;

    public DemoService(HelloWorldService helloWorldService, string firstname, string lastname)
    {
        this.helloWorldService = helloWorldService ?? throw new ArgumentNullException(nameof(helloWorldService));
        this.firstname = firstname ?? throw new ArgumentNullException(nameof(firstname));
        this.lastname = lastname ?? throw new ArgumentNullException(nameof(lastname));
    }

    public string HelloWorld()
    {
        return this.helloWorldService.Hello(firstname, lastname);
    }
}

public class HelloWorldService
{
    public string Hello(string name) => $"Hello {name}";
    public string Hello(string firstname, string lastname) => $"Hello {firstname} {lastname}";
}

// Just a helper method to shorten code registration code
static class ServiceProviderExtensions
{
    public static T ResolveWith<T>(this IServiceProvider provider, params object[] parameters) where T : class => 
        ActivatorUtilities.CreateInstance<T>(provider, parameters);
}

Prints

Output: Hello Tseng Stackoverflow

If you feel uncomfortable with newing the service, you could use the Parameter Object pattern.

So extract the string parameter into its own type

public class ServiceArgs
{
   public string Arg1 {get; set;}
}

And the constructor will now look like

public Service(IOtherService service1, 
               IAnotherOne service2, 
               ServiceArgs args)
{

}

And the setup

_serviceCollection.AddSingleton<ServiceArgs>(_ => new ServiceArgs { Arg1 = ""; });
_serviceCollection.AddSingleton<IOtherService , OtherService>();
_serviceCollection.AddSingleton<IAnotherOne , AnotherOne>();
_serviceCollection.AddSingleton<IService, Service>();

The first benefit is if you need to change the Service constructor and add new services to it, then you don't have to change the new Service(... calls. Another benefit is the setup is a bit cleaner.

For a constructor with a single parameter or two, this might be too much though.

You can inject dependencies with this process also

_serviceCollection.AddSingleton<IOtherService , OtherService>();
_serviceCollection.AddSingleton<IAnotherOne , AnotherOne>();
_serviceCollection.AddSingleton<IService>(x=>new Service( x.GetService<IOtherService>(), x.GetService<IAnotherOne >(), "" ));