Entity Framework POCO SaveChanges() on Update doesn't work?

问题

I'm working with EF CTP 4, using POCO models, adding a new object and call SaveChanges() works but updating the object doesn't work. Here's the code for update:

public void UpdateContact(Contact contact)
        {
            var findContact = GetContact(contact.ContactID);
            findContact = contact;
            _context.SaveChanges();
        }

public Contact GetContact(int contactId)
        {
            return GetAllContacts().SingleOrDefault(c => c.ContactID == contactId);
        }

public IQueryable<Contact> GetAllContacts()
        {
            return _context.Contacts;
        }

I'm not sure what I'm doing wrong here. Any idea? Thanks.

回答1:

The problem is that when you assign findContact = contact the EntityState does not get changed in the ObjectStateManager (so it's still set to Unchanged). Therefore, no Update SQL statement gets generated for the entity. You have several options to do the update:

Option 1: Populate findContact field-by-field:

var findContact = GetContact(contact.ContactID);
findContact.FirstName = contact.FirstName;
findContact.LastName = contact.LastName;
...
_context.SaveChanges();

Option 2: Use the ApplyCurrentValues method:

var findContact = GetContact(contact.ContactID);
_context.ApplyCurrentValues("Contacts", contact);
_context.SaveChanges();

Option 3: Attach the updated entity and change the state in the ObjectStateManager. (Note: this will not make a roundtrip to the database for fetching the existing contact).

_context.AttachTo("Contacts", contact);
var contactEntry = Context.ObjectStateManager.GetObjectStateEntry(contact);
contactEntry.ChangeState(EntityState.Modified);
_context.SaveChanges();

来源：https://stackoverflow.com/questions/4075107/entity-framework-poco-savechanges-on-update-doesnt-work