The Inbox

When writing code outside of actors which shall communicate with actors, the ask pattern can be a solution (see below), but there are two things it cannot do: receiving multiple replies (e.g. by subscribing an IActorRef to a notification service) and watching other actors' lifecycle. For these purposes there is the Inbox class:

var target = system.ActorOf(Props.Empty);
var inbox = Inbox.Create(system);

inbox.Send(target, "hello");

try
{
    inbox.Receive(TimeSpan.FromSeconds(1)).Equals("world");
}
catch (TimeoutException)
{
    // timeout
}

The send method wraps a normal Tell and supplies the internal actor's reference as the sender. This allows the reply to be received on the last line. Watching an actor is quite simple as well

using System.Diagnostics;
...
var inbox = Inbox.Create(system);
inbox.Watch(target);
target.Tell(PoisonPill.Instance, ActorRefs.NoSender);

try
{
    Debug.Assert(inbox.Receive(TimeSpan.FromSeconds(1)) is Terminated);
}
catch (TimeoutException)
{
    // timeout
}