AK2003 - Error

ReceiveActor.Receive message handler delegate must not be a void async delegate. Use ReceiveAsync instead.

Cause

ReceiveActor.Receive 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 System.Threading.Tasks;

public sealed class MyActor : ReceiveActor
{
    public MyActor()
    {
        Receive<int>(async msg =>
            {
                await Task.Yield();
                Sender.Tell(msg);
            });
    }
}

Resolution

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

public sealed class MyActor : ReceiveActor
{
    public MyActor()
    {
        // Use ReceiveAsync() if you're passing an asynchronous delegate
        ReceiveAsync<int>(async msg =>
            {
                await Task.Yield();
                Sender.Tell(msg);
            });
    }
}