order list where top level order is unknown

Question

To order a list with Linq we have to call OrderBy first en call ThenBy on the result for subordinate orderings.

I'm in a situation now where I do not know the top level ordering before hand. I have a list of of orderings which should be applied conditionally.

Like this:

var list = new List<Tuple<int, string, DateTime>>();
list.Add(new Tuple<int, string, DateTime>(1, "B", new DateTime(2020, 1, 1)));
list.Add(new Tuple<int, string, DateTime>(2, "A", new DateTime(2000, 1, 1)));
list.Add(new Tuple<int, string, DateTime>(3, "C", new DateTime(1900, 1, 1)));

var orderedList = list;

if (sortByString)
{
    orderdedList = orderedList.ThenBy(listItem => listItem.Item2);
}

if (sortByDateTime)
{
    orderedList = orderedList.ThenBy(listItem => listItem.Item3);
}

orderList = orderedList.ThenBy(listItem => listItem.Item1);

So the list will always be ordered by Item1, and conditionally by Item2 and/or Item3 first.

How to accomplish this in C#? Solutions without Linq are also welcome.

Answer 1

Just use

var orderedItems = list.OrderBy(_ => 1);

This gives you the default (non-)ordering, and allows you to add as many other orderings as you want afterwards using just ThenBy.

EDIT:

As Tim noted, this does carry a performance penalty - it seems that the default LINQ-to-Objects provider isn't smart enough to rebuild the ordering to get rid of the "non-ordering". This is not a problem if your list is small, but if it's taking non-negligible amount of time, you probably want to do this the hard way.

For example, you could use a helper method like

public static IEnumerable<T> AppendOrdering<T, U>(this IEnumerable<T> @this, 
                                                  Func<T, U> selector)
{
  if (@this is IOrderedEnumerable<T>) return @this.ThenBy(selector);

  return @this.OrderBy(selector);
}

This isn't exactly the same you're doing, but unless you're working on an enumerable that has been ordered before, it's going to work the same way.