Simple Injector DI container nested scopes

Question

There is a Console Application. There are 3 services:

public class SingletonService
{
    public SingletonService()
    {
        Console.WriteLine("SingletonService constructor");
    }
}

public class AppScopeService
{
    public AppScopeService(SingletonService singletonService)
    {
        Console.WriteLine("AppScopeService constructor");
    }
}

public class InnerScopeService
{
    public InnerScopeService(SingletonService singletonService, AppScopeService appScopeService)
    {
        Console.WriteLine("InnerScopeService constructor");
    }
}

I use Simple Injector DI container to create the services:

public class ScopeExperiment
{
    public ScopeExperiment()
    {
        var container = new Container();
        container.Options.DefaultScopedLifestyle = ScopedLifestyle.Flowing;
        container.Register<SingletonService>(Lifestyle.Singleton);
        container.Register<AppScopeService>(Lifestyle.Scoped);
        container.Register<InnerScopeService>(Lifestyle.Scoped);

        
        using (var appScope = new Scope(container))
        {
            Console.WriteLine("<------app scope----->");
            var app = appScope.GetInstance<AppScopeService>();

            using (var innerScope = new Scope(container))
            {
                Console.WriteLine("<------inner scope----->");
                var innerService = innerScope.GetInstance<InnerScopeService>();
            }

            using (var innerScope = new Scope(container))
            {
                Console.WriteLine("<------inner scope 2----->");
                var innerService = innerScope.GetInstance<InnerScopeService>();
            }
        }
    }
}

Run: Program>new ScopeExperiment();

Result in console:

<------app scope----->

SingletonService constructor

AppScopeService constructor

InnerScopeService constructor

<------inner scope----->

AppScopeService constructor

InnerScopeService constructor

<------inner scope 2----->

AppScopeService constructor

InnerScopeService constructor

Expected result:

<------app scope----->

SingletonService constructor

AppScopeService constructor

InnerScopeService constructor

<------inner scope----->

InnerScopeService constructor

<------inner scope 2----->

InnerScopeService constructor

"InnerScopeService" injects "AppScopeService" in constructor. The problem is that "AppScopeService" is created every time "InnerScopeService" is created. Desired behavior is to create "AppScopeService" only once per "appScope".

1. Is there any way to explain to DI container that I want to get dependency from the outer scope (appScope).
2. Does simpleInjector DI container supports nested scopes?

Answer 1

Nested scopes is not a feature that is supported out of the box, but can be implemented, for instance, using the code below.

NOTE: You need to add an abstraction for AppScopeService, as shown in the code below:

There are 3 services:

public class SingletonService
{
    public SingletonService()
    {
        Console.WriteLine("SingletonService constructor");
    }
}

public interface IAppScopeService { }

// NOTE: AppScopeService now implements IAppScopeService
public class AppScopeService : IAppScopeService
{
    public AppScopeService(SingletonService singletonService)
    {
        Console.WriteLine("AppScopeService constructor");
    }
}

public class InnerScopeService
{
    // NOTE: InnerScopeService now depends on IAppScopeService
    public InnerScopeService(
        SingletonService singletonService,
        IAppScopeService appScopeService)
    {
        Console.WriteLine("InnerScopeService constructor");
    }
}

create the services:

public ScopeExperiment()
{
    var container = new Container();
    container.Options.EnableAutoVerification = false;
    container.Options.DefaultScopedLifestyle = ScopedLifestyle.Flowing;
    container.Register<SingletonService>(Lifestyle.Singleton);
    container.Register<InnerScopeService>(Lifestyle.Scoped);

    container.Register<AppScopeService>(Lifestyle.Scoped);
    container.Register<IAppScopeService>(() =>
    {
        var scope = container.GetInstance<Scope>();
        return (scope.GetItem("Parent") as Scope ?? scope)
            .GetInstance<AppScopeService>();
    });

    using (var appScope = new Scope(container))
    {
        Console.WriteLine("<------app scope----->");
        var app = appScope.GetInstance<AppScopeService>();

        using (var innerScope = new Scope(container))
        {
            innerScope.SetItem("Parent", appScope);
            Console.WriteLine("<------inner scope----->");
            var innerService = innerScope.GetInstance<InnerScopeService>();
        }

        using (var innerScope = new Scope(container))
        {
            innerScope.SetItem("Parent", appScope);
            Console.WriteLine("<------inner scope 2----->");
            var innerService = innerScope.GetInstance<InnerScopeService>();
        }
    }
}

Result in console:

<------app scope----->
SingletonService constructor
AppScopeService constructor
<------inner scope----->
InnerScopeService constructor
<------inner scope 2----->
InnerScopeService constructor