Implementing a state pattern

Question

I have an object that based on it's properties, will perform different operations in it's service layer.

Let's say it's a customer with the following states: Registered, RegisteredWithoutContactOnFile, and NoRegistration.

These states are based on properties set on the object. So it would be something like the following:

public class Customer
{
        public Registration Registration {get; set;}
        public string Email {get; set;}
        public int Id {get; set;}
}

Then there would be the CustomerService which does different database operations based on what is entered like such:

public class CustomerService
{
        public void RegisterCustomer(Customer customer)
        {
               if (customer.Registration != null && customer.Id == 0) //Registered without contact -- insert a new contact into the database

               else if (customer.Registration == null && customer.Id == 0) // No registration/contact -- insert both a registration and contact into the database
               
               else if (customer.Registration != null && customer.Id > 0) //Registered and contact on file -- update contact and registration information in the database
}

I basically want to get rid of this if/else statement in the service layer and make it a state in my object so that I can more easily test it. Can anyone assist with how I would go about this or if I'm going about this the wrong way? I will say I am interested in the state pattern as I have been working on Finite State Machines for an AI agent but I feel like it makes sense here as well. Perhaps a strategy pattern would work? In short, I'm just trying to get the logic out of the service layer to make it more testable... any insight would be greatly appreciated!

Answer 1

You can't have a state machine without transitions and new states. This looks more like validation checks. It can be implemented used pattern matching and a switch statement, eg :

switch (customer.Registrarion,customer.Id)
{
    case (null,0):
        //No Registration, contact:
        break;
    case (Registration r,0):
        //Registered without contact 
        break;
    case (Registration r, _):
        //Registered with contact 
        break;
    defaut:
        throw new InvalidOperationException("Invalid state!");
}

Pattern matching with switch expressions can be used to implement state machines, like this example from the docs :

var newState = (state, operation, key.IsValid) switch
{
  (State.Opened, Operation.Close, _)      => State.Closed,
  (State.Opened, Operation.Open, _)       => throw new Exception(
    "Can't open an opened door"),
  (State.Opened, Operation.Lock, true)    => State.Locked,
  (State.Locked, Operation.Open, true)    => State.Opened,
  (State.Closed, Operation.Open, false)   => State.Locked,
  (State.Closed, Operation.Lock, true)    => State.Locked,
  (State.Closed, Operation.Close, _)      => throw new Exception(
    "Can't close a closed door"),
  _ => state
};

This expression uses the original state, an operation and a parameter to decide the new state

Answer 2

ugh...

refactor multi-if or nested-if statement can have many different ways, something like you could use a template pattern, i.e. translate your customer to different type by states, and then have a dictionary of [, ], the core business logic becomes

handlers[customer.GetType()].Handle(customer) 

another way is instead of using if-statement, you could have concrete type (of each state) and then pattern matching.

one more interesting way is employing an actor to handle this. An actor comes with a nature built-in FSM.

What you can do is issuing a message, your handling actor calls Becomes, which enters in a state that will only handle the messages under that state.