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);
  });
}