Drive an Aggreagte View Model by setting properties or calling methods?

Question

I'm debating with myself about the best practice approach to controlling an Aggreate View Model class that I've created for my app... Let's say I have an aggregate model that has a PurchaseOrder object and a list of line items that belong to that Purchase Order, and a few other auxuliary/related objects. This view model is just a wrapper around all these objects that you would typcially need when working on any give PurchaseOrder.

After after creating an instance of this view model, I then want it to load up a PurchaseOrder (and it will load the PurchaseOrderLineItems automatically and saturate all the other related objects)...

So, to instruct the view model to load up a PurchaseOrder, is it more acceptable to:

1. Instruct the view model by setting a property on it (and let the property setter of the view model class respond by loading up the data)

   ViewModel.PoNo = 1234;

or

1. Call a method on the view model to do the work:

   ViewModel.LoadPurchaseOrder(1234);

Just to give a few mode detials about the Aggregate View Model, it basically looks like this:

    public class ViewModel
    {
      //-- private fields
      PurchaseOrder _Po = new PurcaseOrder();
      List<PurchaseOrderItem> _PoLineItems;
      Vendor _Vend = new Vendor();
      int _PoNo;

      //-- public properties here

      ViewModel(){} // Constructor

    }

Answer 1

Does this ViewModel serve any purpose other than relating all the PurchaseOrder informaton together? If not i would say you should pass your purchase order in the constructor of ViewModel because it seems like ViewModel would only be in a valid state if it had a PurchaseOrder.

EDIT: Given the 2 options you have listed, I think a method call makes more sense than setting a property as it is easier to tell that you are loading a PurchaseOrder into this ViewModel. As a developer, I wouldn't think that setting an integer property would end up loading all kinds of objects on the ViewModel, but one might expect that from calling a method.