The relationship between the two objects cannot be defined

The relationship between the two objects cannot be defined

I want add and edit items in Users. I use this code.
DataContext db = new DataContext();
private void Save()
{
User user = (SelectedUser == null) ? new User() :
db.User.Find(SelectedUser.UserName);
user.FirstName = FirstName;
user.CcRowIndex = CcRowIndex;
user.Image = Image;
user.LastName = LastName;
user.OrganizationalPostId = OrganizationalPost;
user.OrganizationalUnitId = OrganizationalUnit;
user.Password = Password;
user.Signature = Signature;
user.SubsetUsers = SubsetUsersList;
user.UserName = UserName;
if (SelectedUser == null)
{
db.User.Add(user);
}
db.SaveChanges();
}
I add Items, but when i edit items i get error
The relationship between the two objects cannot be defined because they
are attached to different ObjectContext objects.+ savechange of
datacontext