AK2005 - Error

ReceivePersistentActor.Command message handler delegate must not be a void async delegate. Use ReceiveAsync instead.

Cause

ReceivePersistentActor.Command accepts an Action<TMessage> as a delegate, any void async delegate passed as an argument will be invoked as a detached asynchronous function that can cause erroneous message processing behavior.

An example:

using Akka.Actor;
using Akka.Persistence;
using System.Threading.Tasks;

public class MyActor: ReceivePersistentActor
{
    public MyActor(string persistenceId)
    {
        PersistenceId = persistenceId;
        
        Command<int>(async msg =>
        {
            await Task.Yield();
        });
    }
    
    public override string PersistenceId { get; }
}

Resolution

using Akka.Actor;
using Akka.Persistence;
using System.Threading.Tasks;

public class MyActor: ReceivePersistentActor
{
    public MyActor(string persistenceId)
    {
        PersistenceId = persistenceId;
        
        // Use CommandAsync() if you're passing an asynchronous delegate
        CommandAsync<int>(async msg =>
        {
            await Task.Yield();
        });
    }
    
    public override string PersistenceId { get; }
}