c# - Can I split an IEnumerable into two by a boolean criteria without two queries?

Question

Can I split an IEnumerable<T> into two IEnumerable<T> using LINQ and only a single query/LINQ statement?

I want to avoid iterating through the IEnumerable<T> twice. For example, is it possible to combine the last two statements below so allValues is only traversed once?

IEnumerable<MyObj> allValues = ...
List<MyObj> trues = allValues.Where( val => val.SomeProp ).ToList();
List<MyObj> falses = allValues.Where( val => !val.SomeProp ).ToList();

Answer 1

You can use this:

var groups = allValues.GroupBy(val => val.SomeProp);

To force immediate evaluation like in your example:

var groups = allValues.GroupBy(val => val.SomeProp)
                      .ToDictionary(g => g.Key, g => g.ToList());
List<MyObj> trues = groups[true];
List<MyObj> falses = groups[false];