C# How to split a List in two using LINQ

You can use ToLookup extension method as follows:

 List<Foo> items = new List<Foo> { new Foo { Name="A",Condition=true},new Foo { Name = "B", Condition = true },new Foo { Name = "C", Condition = false } };

  var lookupItems = items.ToLookup(item => item.Condition);
        var lstTrueItems = lookupItems[true];
        var lstFalseItems = lookupItems[false];

GroupBy and Single should accomplish what you're looking for:

var groups = events.GroupBy(e => e.Closer_User_ID == null).ToList(); // As others mentioned this needs to be materialized to prevent `events` from being iterated twice.
var openEvents = groups.SingleOrDefault(grp => grp.Key == true)?.ToList() ?? new List<EventModel>();
var closedEvents = groups.SingleOrDefault(grp => grp.Key == false)?.ToList() ?? new List<EventModel>();

One line solution by using ForEach method of List:

List<EventModel> events = GetAllEvents();

List<EventModel> openEvents = new List<EventModel>();
List<EventModel> closedEvents = new List<EventModel>();

events.ForEach(x => (x.Closer_User_ID == null ? openEvents : closedEvents).Add(x));