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

Question

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

I want to avoid iter

Answer 1

Some people like Dictionaries, but I prefer Lookups due to the behavior when a key is missing.

IEnumerable allValues = ...
ILookup theLookup = allValues.ToLookup(val => val.SomeProp);

  //does not throw when there are not any true elements.
List trues = theLookup[true].ToList();
  //does not throw when there are not any false elements.
List falses = theLookup[false].ToList();

Unfortunately, this approach enumerates twice - once to create the lookup, then once to create the lists.

If you don't really need lists, you can get this down to a single iteration:

IEnumerable trues = theLookup[true];
IEnumerable falses = theLookup[false];