.NET Core - Class configured in container with Addsingleton() needs to be a Singleton?

Question

A Web API application in . NET Core consumes a DLL in . NET Standard. DLL has a Foo class that depends on the Bar class:

class Foo
{
    private readonly IBar bar;
    
    public Foo(IBar bar)
    {
        this.bar = bar;
    }
}

Bar is not a Singleton (no private constructor and no .Getinstance()), but must have only one instance shared throughout the application:

class Bar : IBar
{
    public int Contador {get; set;}
}

To do so, the Web API container is configured as follows:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<IBar, Bar>();
}

Now a Web application in . NET Framework 4.6.1 also needs to consume this DLL. This application has no container configuration. Follow the questions:

1. When instantiating the Foo class, how to ensure that the injected Bar class behaves like Singleton? The Bar class should have been implemented as Singleton from the start?
2. Class injected via Addsingleton() needs to be a Singleton?
3. The configuration of the Bar class in the Web API container, if it was a Singleton, would be as below?

public void Configureservices(Iservicecollection services) { services.Addsingleton<Ibar, Bar.Getinstance()>(); }

Answer 1

1. For the Bar class to behave like Singleton just use the method AddSingleton when configuring API services. You don’t need to create Getinstance in the private class or constructs this is automatically handled by the framework.

2. The injected class via Singleton can be a "normal" class. You do not need to follow the Singleton standard as if you were doing it in your hand. Create the public constructor with any dependencies that need to be solved or not and the methods it contains. The lifetime of the object will be set when you are setting up the service using the different methods: AddSingleton, AddScoped, AddTransient

3. To add as Singleton, do: services.AddSingleton<IBar, Bar>();. A single instance of this class will be created throughout the application time.