Entity Framework as DAL how to implement Update and Delete correctly - entity-framework

Question

I'm writing a DAL class using EF4.0, I've read
http://www.codeproject.com/Articles/43367/ADO-NET-Entity-Framework-as-Data-Access-Layer
and
http://msdn.microsoft.com/en-us/magazine/cc700340.aspx
But when I test their code, I meet some problem with the Update and Delete method.
The DAL class all code is below:
public class FriendlinkDA : IDisposable
{
private EdiBlogEntities context;
public FriendlinkDA()
{
context = new EdiBlogEntities();
}
public void Dispose()
{
context.Dispose();
}
public FriendLink GetFriendLink(Guid id)
{
return context.FriendLink.FirstOrDefault(f => f.Id == id);
}
public void Update(FriendLink model)
{
// Way 1: (throw exception)
//context.Attach(model);
//model.SetAllModified(context);
//context.SaveChanges();
// Way 2:
EntityKey key;
object originalItem;
key = context.CreateEntityKey("FriendLink", model);
if (context.TryGetObjectByKey(key, out originalItem))
{
context.ApplyCurrentValues(key.EntitySetName, model);
//context.ApplyPropertyChanges(key.EntitySetName, model);
}
context.SaveChanges();
}
public void Delete(FriendLink model)
{
// Way 1:
context.Attach(model);
context.DeleteObject(model);
context.SaveChanges();
// Way 2:
//var item = context.FriendLink.FirstOrDefault(f => f.Id == model.Id);
//context.DeleteObject(item);
//context.SaveChanges();
}
}
The extension method is:
public static void SetAllModified<T>(this T entity, ObjectContext context) where T : IEntityWithKey
{
var stateEntry = context.ObjectStateManager.GetObjectStateEntry(entity.EntityKey);
var propertyNameList = stateEntry.CurrentValues.DataRecordInfo.FieldMetadata.Select
(pn => pn.FieldType.Name);
foreach (var propName in propertyNameList)
stateEntry.SetModifiedProperty(propName);
}
In the application, I am use the DAL like this:
// Delete
using (var optFriendlink = new FriendlinkDA())
{
var test = optFriendlink.GetFriendLink(new Guid("81F58198-D396-41DE-A240-FC306C7343E8"));
optFriendlink.Delete(test);
}
// Update
using (var optFriendlink = new FriendlinkDA())
{
var testLink = optFriendlink.GetFriendLink(new Guid("62FD0ACF-40C3-4BAD-B438-38BB540A6080"));
testLink.Title = "ABC";
optFriendlink.Update(testLink);
}
Question 1:
In Delete(), both way 1 and way 2 can work. Which one is better?
Question 2:
In Update(), way 1 give me an exception: The object cannot be attached because it is already in the object context. An object can only be reattached when it is in an unchanged state.
on this statment: context.Attach(model);
but way 2 is fine.
why is this happening? I also attach the model in Delete(), why Delete() is working fine? how I can write the update correctly?

Answer 1

The exception says it all:
An object can only be reattached when it is in an unchanged state.
You change the object in the code snippet under // Update, so that's why it cannot be re-attached.
As to which method is better. Normally you would get an object from a context, dispose of the context, do something with the object and then use a new context to save the object. In that case using Attach is much more comfortable then getting an object by Id first.