Why does Props.Create use Activator even when passing an expression?

[chylex]

The Props Create<TActor>(Expression<Func<TActor>> factory) overload takes an Expression, which if it is a NewExpression, creates Props with an ActivatorProducer. When I use the recommended static factory method, it makes sense to me (and Rider recommends this) to make the actor constructor private. However, this crashes at runtime, because Activator cannot call private constructors.

This is really confusing to me, so I'm wondering what's the rationale?

Normally in .NET, if an Expression calls a private constructor, compiling it and calling it elsewhere works - except in Akka.NET - making Akka.NET's behavior counter-intuitive.

It's also not clear why use an Expression at all, as opposed to a Func. The Props class already includes a FactoryConsumer implementation that works with a Func, but as far as I can tell, FactoryConsumer is not used anywhere.

I might try copying FactoryConsumer and using it via Props.CreateBy(IIndirectActorProducer), so I can make actor constructors private and avoid Activator, but I guess there's a good reason why Akka.NET doesn't work this way by default?

[to11mtm]

Do you happen to have a repro? I can look into this more when back from Vacation to see if we can enhance the expression handling in some way... As far as the reason for an expression rather than a function, my general assumption is that because Props are intended to be serializable, an expression is typically easier to decompose into a constructor and args. That said, we may be able to add some form of static factory for the parser (we already have some custom parse logic to try and avoid .Compile()) but an example case will make it easier per above.

…

On Mon, Feb 26, 2024, 2:31 PM chylex ***@***.***> wrote: The Props Create<TActor>(Expression<Func<TActor>> factory) overload takes an Expression, which if it is a NewExpression, creates Props with an ActivatorProducer. When I use the recommended static factory method, it makes sense to me (and Rider recommends this) to make the actor constructor private. However, this crashes at runtime, because Activator cannot call private constructors. This is really confusing to me, so I'm wondering what's the rationale? Normally in .NET, if an Expression calls a private constructor, compiling it and calling it elsewhere works - except in Akka.NET - making Akka.NET's behavior counter-intuitive. It's also not clear why use an Expression at all, as opposed to a Func. The Props class already includes a FactoryConsumer implementation that works with a Func, but as far as I can tell, FactoryConsumer is not used anywhere. I might try copying FactoryConsumer and using it via Props.CreateBy, so I can make actor constructors private and avoid Activator, but I guess there's a good reason why Akka.NET doesn't work this way by default? — Reply to this email directly, view it on GitHub <#7104>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AC7UYVJJ5LQRLPB3XS3BSATYVTPIRAVCNFSM6AAAAABD2ZFPNSVHI2DSMVQWIX3LMV43ERDJONRXK43TNFXW4OZWGI3TONBSGU> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[chylex]

Minimal repro:

public class Program {
	public static void Main() {
		using var actorSystem = ActorSystem.Create("Test");
		actorSystem.ActorOf(Actor.Factory(5));
	}
}

sealed class Actor : ReceiveActor {
	public static Props Factory(int value) {
		return Props.Create(() => new Actor(value));
	}

	private int Value { get; }

	private Actor(int value) {
		Value = value;
	}
}

It should be basically the same as https://getakka.net/articles/actors/receive-actor-api.html#recommended-practices, except with a private constructor. With the private constructor, Akka.NET throws at runtime:

[ERROR][02.26.2024 21:44:08.850Z][Thread 0022][akka://Test/user/$a]
Error while creating actor instance of type Controller.Actor with 1 args: (5)
Cause: [akka://Test/user/$a#467941579]: Akka.Actor.ActorInitializationException: Exception during creation                                               
 ---> System.TypeLoadException: Error while creating actor instance of type Controller.Actor with 1 args: (5)   
 ---> System.MissingMethodException: Constructor on type 'Controller.Actor' not found.                          
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)
   at System.Activator.CreateInstance(Type type, Object[] args)                                                         
   at Akka.Actor.Props.ActivatorProducer.Produce()                                                                      
   at Akka.Actor.Props.NewActor()                                                                                       
   --- End of inner exception stack trace ---                                                                           
   at Akka.Actor.Props.NewActor()                                                                                       
   at Akka.Actor.ActorCell.CreateNewActorInstance()                                                                     
   at Akka.Actor.ActorCell.<>c__DisplayClass116_0.<NewActor>b__0()                                                      
   at Akka.Actor.ActorCell.UseThreadContext(Action action)                                                              
   at Akka.Actor.ActorCell.NewActor()                                                                                   
   at Akka.Actor.ActorCell.Create(Exception failure)                                                                    
   --- End of inner exception stack trace ---                                                                           
   at Akka.Actor.ActorCell.Create(Exception failure)                                                                    
   at Akka.Actor.ActorCell.SysMsgInvokeAll(EarliestFirstSystemMessageList messages, Int32 currentState)    

[chylex]

Personally I don't need serialization, so making a custom IIndirectActorProducer is good enough for me. This answers my question, but should I mark it as the answer now, or keep it open until you've had a chance to look at the repro example?

[Aaronontheweb]

Glad you were able to find the IIndirectActorProducer - that's what I was going to suggest! I don't think we'll be making any significant changes to Props (as far as NewExpression usage goes) any time soon since that'd break features like remote actor deployment, but I'll mark your solution as the answer here in case other users run into this.

[chylex]

Cool; I already mentioned IIndirectActorProducer indirectly (heh) in my original post, but I'll edit to make it clearer and mark to11mtm's response as the answer since that directly answers the "why" in my original question.

Thanks to both for answering so quickly!

[chylex]

I have opened an improvement for Rider (https://youtrack.jetbrains.com/issue/RIDER-106480) to stop suggesting making actor constructors private. If you find a way to allow private constructors, please let me know so I can update the issue.