public class MapMemberIDSet 

await QueuedTask.Run(() =>
{
  MapMemberIDSet rootNodes = MapView.Active.GetRootNodes();
  var rootNodeDict = rootNodes.ToDictionary();

  // rootNodeDict is a Dictionary<MapMember, List<long>>

  // access a particular mapMember in the Dictionary
  if (rootNodeDict.ContainsKey(mapMember))
  {
    var oids = rootNodeDict[mapMember];
  }

  // OR iterate through the dictionary
  foreach (var (mm, oids) in rootNodeDict)
  {
    // do something
  }
});

await QueuedTask.Run(() =>
{
  var dict = new Dictionary<MapMember, List<long>>();
  dict.Add(entityLayer, oids);
  MapMemberIDSet mmIDSet = MapMemberIDSet.FromDictionary(dict);

  MapView.Active.SetRootNodes(mmIDSet);
});

await QueuedTask.Run(() =>
{
  var mapSel = MapView.Active.Map.GetSelection();

  MapView.Active.SetRootNodes(mapSel);
});

await QueuedTask.Run(() =>
{
  var dict = new Dictionary<MapMember, List<long>>();
  dict.Add(entityLayer, oids);
  dict.Add(entityLayer2, oids2);
  MapMemberIDSet mmIDSet = MapMemberIDSet.FromDictionary(dict);

  MapView.Active.SelectRootNodes(mmIDSet);
});