Programming Entity Framework - Cdn.oreilly.com

able to tap into the same set of services that the EntityObject has access to. The
POCO class now has access to these services and can therefore interact with the
ObjectContext in a similar fashion as the EntityObject.
Synchronizing Relationships by Proxy
Finally, we can return to the third method of fixing up two-way relationships. With
proxies, this also benefits classes with both a foreign key and related navigation property
instance (e.g., Address.ContactID and Address.Contact) because the proxy
will synchronize them. You may recall seeing EntityObjects do this in Chapter 10.
First let’s look at a scenario where you are linking two existing entities. The following
code queries for a random Contact and an Address and joins them:
Address address = context.Addresses.
Where(a=>a.City=="Winnipeg").FirstOrDefault();
Contact contact = context.Contacts.FirstOrDefault();
contact.Addresses.Add(address);
If you are not using the proxy behavior (i.e., the properties are not marked as virtual),
then after this code is run, address.Contact and address.ContactID will be
null.
If you have enabled the proxy to work, address.Contact will point to the contact
and address.ContactID will have the correct value.
If you are creating new objects and you want the relationships to be fixed up there is
another important rule to know about.
You might just create a new address by instantiating it:
Address address = new Address();
The context will have absolutely no clue about this address, and if you added it to
contact.Addresses, you would not get the fix-up behavior.
You need to let the context instantiate the object for you:
Address address = context.CreateObject();
Then when you add this address to the collection, or set address.Contact to the
existing contact, the relationship and foreign key will be automatically fixed.
If you are joining two new objects that were created with CreateObject, you will still
get the fix-up behavior, but remember that the foreign key value (e.g., ContactID) will
be 0 since it is unassigned. But that is still different from null, which is what you would
get when the fix-up is not occurring at all.
The Critical Rules for Getting Proxy
Behavior with POCO Objects
I pointed out three critical rules in the previous text that are worthy of
highlighting along with some others that are equally important.
Rule 1: To get the proxy behavior for a POCO object, every single property
(scalar and navigation properties) must be made virtual and public using the C#
virtual keyword or the VB Overridable keyword.