Singleton dependencies are re-created in every dependent class

[FabioCastle]

I found out that using Dependency Injection to share a singleton service between classes doesn't work as expected.

Please, consider this simple example of PlugIn that inject the same singleton service to both a controller and an IHostedService:

using Microsoft.Extensions.DependencyInjection;
using TauriDotNetBridge.Contracts;

public class PlugIn : IPlugIn
{
    public void Initialize(IServiceCollection services)
    {
        services.AddSingleton<SharedService>();
        services.AddSingleton<DemoController>();
        services.AddSingleton<IHostedService, DemoService>();
    }
}

The SharedService class holds a reference to a common object that should be accessed both by DemoController and DemoService:

public class SharedService
{
    public SharedData? SharedData { get; set; }
}

public record SharedData(bool Initialized = false);

public class DemoController(SharedService _sharedService)
{
    public SharedData? GetData()
    {
        return _sharedService?.SharedData;
    }
}

public class DemoService(SharedService _sharedService) : IHostedService
{
    public async Task StartAsync(CancellationToken cancellationToken)
    {
        await Task.Delay(1000, cancellationToken);
        _sharedService.SharedData = new SharedData(true);

        Console.WriteLine("Shared data initialized");
    }
}

The expected behaviour is that the service should wait 1 second and then initialize the SharedData instance with a "true" value.
After that, the controller should return the JSON:

{
    "initialized": true
}

But the result will always be:

{
    "initialized": false
}

I tried to call Console.WriteLine("SharedService created") in the constructor of SharedService class and I can tell that the controller and the service will use a different instance, so that the controller can't access the initialized data.

As a workaround it's possible to define the class SharedData following the singleton pattern and make the controller and the service call static methods on it, but since I could use DI in the PlugIn class I expected to have the correct behaviour in handling this situation.