When using the Receive actor API in Akka.NET, we can take advantage of overloads of the Receive method that allow us to specify predicates.

A message will now be handled if it is of the correct type and the predicate is met.

Take the following actor that sends emails with a different message if a customer is “high value”:

class WelcomeEmailSender : ReceiveActor
{
    public WelcomeEmailSender()
    {
        Receive<SendNewCustomerWelcomeEmail>(message => HandleMessage(message));
    }

    private void HandleMessage(SendNewCustomerWelcomeEmail message)
    {            
        if (message.IsHighValueCustomer)
        {
            SendEmail("Dear high value customer...", message.EmailAddress);
        }
        else
        {
            SendEmail("Yo! ...", message.EmailAddress);
        }
    }

    private void SendEmail(string greeting, string emailAddress)
    {
        Console.WriteLine("{0} to {1}", greeting, emailAddress);
    }
}

In the preceding code, the if statement is deciding what action is taken – the welcome message to be sent.

Using an overload of the receive method, we can add a predicate that replaces this if statement:

class WelcomeEmailSender2 : ReceiveActor
{
    public WelcomeEmailSender2()
    {
        Receive<SendNewCustomerWelcomeEmail>(
            message =>
            {
                SendEmail("Dear high value customer...", message.EmailAddress);
            },
            handleIf => handleIf.IsHighValueCustomer);

        Receive<SendNewCustomerWelcomeEmail>(
            message =>
            {
                SendEmail("Yo! ...", message.EmailAddress);
            }, 
            handleIf => handleIf.IsHighValueCustomer == false);
    }

    private void SendEmail(string greeting, string emailAddress)
    {
        Console.WriteLine("{0} to {1}", greeting, emailAddress);
    }
}

In the preceding code the predicate (represented by the lambda: handleIf => handleIf.IsHighValueCustomer) replaces the if.

In these example we only have two possible predicates because we are looking at the boolean “IsHighValueCustomer”. The following code shows predicates being defined for non-boolean (string) values:

class SomeActor : ReceiveActor
{
    public SomeActor()
    {
        Receive<string>(message => Console.WriteLine("Knock knock..."), handleIfMessage => handleIfMessage.Equals("Tell me a joke"));
        Receive<string>(message => Console.WriteLine("Hello There!"), handleIfMessage => handleIfMessage.Equals("Say Hi"));
        Receive<string>(message => Console.WriteLine("Woof"), handleIfMessage => handleIfMessage.Equals("Bark like a dog"));
    }
}

If we were to send this actor the following messages they would be received and handled:

IActorRef someActor = actorSystem.ActorOf<SomeActor>("Some");

someActor.Tell("Tell me a joke");
someActor.Tell("Say Hi");
someActor.Tell("Bark like a dog");

However, the following would not be received as it has no matching predicate (even though the message is still of type string):

someActor.Tell("Say bye");

To learn more about Akka.NET, check out the docs or my Pluralsight courses: Building Concurrent Applications with the Actor Model in Akka.NET and Implementing Logging and Dependency Injection in Akka.NET.

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