Detect Change in EntityFrameWork - c#-4.0

Question

In my current project I need write in a table all values are changed in the application.
Ex. the guy update the UserName, I need put in a table UserName old value "1" new value "2".
I tried use the ObjectStateEntry but this return all fields. I think the FW return all because my code.
public USER Save(USER obj)
{
using(TPPTEntities db = new TPPTEntities())
{
db.Connection.Open();
USER o = (from n in db.USERs where n.ID == obj.ID select n).FirstOrDefault();
if (o == null)
{
o = new USER()
{
BruteForce = 0,
Email = obj.Email,
IsBlock = false,
LastLogin = DateTime.Now,
Name = obj.Name,
UserName = obj.UserName,
UserPassword = new byte[0],
};
db.AddToUSERs(o);
}
else
{
o.Email = obj.Email;
o.Name = obj.Name;
o.UserName = obj.UserName;
}
db.SaveChanges();
db.Connection.Close();
}
return obj;
}

Answer 1

A way to get old and new values is this:
var ose = this.ObjectStateManager.GetObjectStateEntry(o.EntityKey);
foreach (string propName in ose.GetModifiedProperties())
{
string.Format("Property '{0}', old value: {1}, new value: {2}",
propName, ose.OriginalValues[propName], ose.CurrentValues[propName]);
}
This is pretty useless, of course, but I'm sure you'll know what to do in the foreach loop to store the changes.

Answer 2

Is this a WCF Service? In that case, the changes will probably never come trough since changes to the Object Graph are made where the Object Context is not available. Consider using Self-Tracking Entities