How can I compare to C# collections and issue Add, Delete commands to make them equal?

How can I compare to C# collections and issue Add, Delete commands to make
them equal?

I have two ICollection collections:
public partial class ObjectiveDetail
{
public int ObjectiveDetailId { get; set; }
public int ObjectiveId { get; set; }
public int Number { get; set; }
public string Text { get; set; }
public virtual Objective Objective { get; set; }
}
var _objDetail1: // contains a list of ObjectiveDetails from my database.
var _objDetail2: // contains a list of ObjectiveDetails from web front end.
If there is a record present in the first list but not the second then I
would like to:
_uow.ObjectiveDetails.Delete(_objectiveDetail);
If there is a record present in the second list but not the first then I
would like to:
_uow.ObjectiveDetails.Add(_objectiveDetail);
I was thinking to do this with some kind of:
foreach (var _objectiveDetail in _objectiveDetails) {}
but then I think I might need to have two of these and I am also wondering
if there is a better way. Does anyone have any suggestions as to how I
could do this?