Update Entity Framework object from another object of same type

Question

Is there any method to update an object from a strongly typed object without listing each field ?
Lets consider the following case:

    using (var context = new MyDBEntities())
{
     var user = (User)Session["EditedUser"];
     var oldUser = context.Users.FirstOrDefault(o => o.Id == user.Id);
     oldUser.FirstName= user.FirstName;
     oldUser.LastName = user.LastName;
     etc ....
     context.SaveChanges();
}

I have 29 more fields and I am not willing to write them down one by one every time. What I am looking for should be similar to this

using (var context = new MyDBEntities())
{
     var user = (User)Session["EditedUser"];
     var oldUser = context.Users.FirstOrDefault(o => o.Id == user.Id);
     oldUser=user;
     context.SaveChanges();
}

Which fails for sure because of the entity's primary key violation. What I would like to achieve here is to update all the fields except the primary key value.

Answer 1

Used the Attach function thanks to 'Silvermind' Now my code looks like this

using (var context = new MyDBEntities())
{
    try
    {
        var user = (User)Session["EditedUser"];
        context.Users.Attach(user);
        context.ObjectStateManager.ChangeObjectState(user, EntityState.Modified);
        context.SaveChanges();
        Session["EditedUser"] = null;
        return "ok";
    }
    catch (Exception ex)
    {
        return ex.Message;
    }
}