Register EventHandler in CQRS

Question

I am trying to model purchasing domain using CQRS & DDD, i know that i raise events in domain but i don't know where to Register them when i am using commands. Should event handlers be registered in command handlers? or maybe i misunderstood something. this is my process can you help model it right way?

Finalize purchase order Command is given, than command handler finalizes order (gets order from repository, changes its state and saves back to db), order finalized event occurs in domain model, than event handler finds this order using id with its line items, find it suppliers contact info(may be email or even external service) and notifies him about new purchase order.

My Commands & Command Handlers are in Application Layer(Event Handlers should be here also?). Domain Model, Events and IRepositories in Domain Layer. Repository Implementations in Infrastructure Layer.

Domain Model (skipped most of properties):

public class PurchaseOrder
{
   public PurchaseOrder(int purchaseOrderID, int supplierID, bool isOrderFinalized)
   {
      PurchaseOrderID = purchaseOrderID;
      SupplierID = supplierID;
      IsOrderFinalized = isOrderFinalized;
   }
   public int PurchaseOrderID { get; private set; }
   public int SupplierID { get; private set; }
   public bool IsOrderFinalized { get; private set; }

   public static PurchaseOrder CreateNew(int supplierID)
   {
      return new PurchaseOrder(0, supplierID, false);
   }

   public void FinalizeOrder()
   {
      IsOrderFinalized = true;
      DomainEvents.Raise(new PurchaseOrderFinalized(PurchaseOrderID));
   }
}

FinalizePurchaseOrder Command

public class FinalizePurchaseOrder : ICommand
{
   public FinalizePurchaseOrder (int purchaseOrderID)
   {
      PurchaseOrderID = purchaseOrderID;
   }
   public int PurchaseOrderID { get; private set; }
}

Command Handler

public class PurchaseOrdersCommandHandler :  ICommandHandler<FinalizePurchaseOrder>
{
    public void Handle(FinalizePurchaseOrder command)
    {
        var purchaseOrder = purchaseOrderRepository.FindByID(command.PurchaseOrderID);
        // Should i register event handler here?
        // DomainEvents.Register<PurchaseOrderFinalized>(PurchaseOrderFinalizedHandler);
        purchaseOrder.FinalizePurchaseOrder();
        purchaseOrderRepository.Save(purchaseOrder);
    }
}

Event and Event Handler looks like this:

public class PurchaseOrderFinalized 
{
   public PurchaseOrderFinalized(int purchaserOrderID)
   {
      PurchaseOrderID = purchaseOrderID;
   }
}

public void PurchaseOrderFinalizedHandler (PurchaseOrderFinalized evt)
{
   // TODO: Get PurchaseOrder with its line items, and notify supplier about new order
}

Answer 1

Should event handlers be registered in command handlers?

Not unless they are dynamic, no. You would typically wire them up in your application's CompositionRoot. The idea being that all the wiring up takes place when your app loads and before it is "ready".

Where are you registering your command handlers? You should be registering your event handlers in the same place.

[UPDATE]

For an example, see https://github.com/gregoryyoung/m-r/blob/master/CQRSGui/Global.asax.cs