How to implement Depedency Injection correctly when injected service has required constructor argument

>=RC.5

@NgModule({
  providers: [/*providers*/]
  ...
})

or for lazy loaded modules

static forRoot(config: UserServiceConfig): ModuleWithProviders {
  return {
    ngModule: CoreModule,
    providers: [
      {provide: UserServiceConfig, useValue: config }
    ]
  };
}

When a component or service requests a value from DI like

constructor(someField:SomeType) {}

DI looks up the provider by the type SomeType. If @Inject(SomeType) is added

constructor(@Inject(SomeType) someField:SomeType) {}

However there are situations where you want to customize the behavior for example to inject a configuration setting.

constructor(@Inject('someName') someField:string) {}

A dependency is an object that another object depends on. Examine the following MessageWriter class with a Write method that other classes depend on:

public class MessageWriter
{
    public void Write(string message)
    {
        Console.WriteLine($"MessageWriter.Write(message: \"{message}\")");
    }
}

A class can create an instance of the MessageWriter class to make use of its Write method. In the following example, the MessageWriter class is a dependency of the Worker class:

public class Worker : BackgroundService
{
    private readonly MessageWriter _messageWriter = new MessageWriter();

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            _messageWriter.Write($"Worker running at: {DateTimeOffset.Now}");
            await Task.Delay(1000, stoppingToken);
        }
    }
}

As an example, the IMessageWriter interface defines the Write method:

namespace DependencyInjection.Example;

public interface IMessageWriter
{
    void Write(string message);
}

This interface is implemented by a concrete type, MessageWriter:

namespace DependencyInjection.Example;

public class MessageWriter : IMessageWriter
{
    public void Write(string message)
    {
        Console.WriteLine($"MessageWriter.Write(message: \"{message}\")");
    }
}

The sample code registers the IMessageWriter service with the concrete type MessageWriter. The AddScoped method registers the service with a scoped lifetime, the lifetime of a single request. Service lifetimes are described later in this article.

namespace DependencyInjection.Example;

class Program
{
    static Task Main(string[] args) =>
        CreateHostBuilder(args).Build().RunAsync();

    static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureServices((_, services) =>
                services.AddHostedService<Worker>()
                        .AddScoped<IMessageWriter, MessageWriter>());
}

Or, you could added using the NuGet command line or edit the *.csproj file directly, since this is perfectly acceptable in the .NET Core era.

<ItemGroup>
    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="1.1.1" />
  </ItemGroup>