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<MyObj> allValues = ...
ILookup<bool, MyObj> theLookup = allValues.ToLookup(val => val.SomeProp);

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

Answer 2

Copy pasta extension method for your convenience.

public static void Fork<T>(
    this IEnumerable<T> source,
    Func<T, bool> pred,
    out IEnumerable<T> matches,
    out IEnumerable<T> nonMatches)
{
    var groupedByMatching = source.ToLookup(pred);
    matches = groupedByMatching[true];
    nonMatches = groupedByMatching[false];
}

Or using tuples in C# 7.0

public static (IEnumerable<T> matches, IEnumerable<T> nonMatches) Fork<T>(
    this IEnumerable<T> source,
    Func<T, bool> pred)
{
    var groupedByMatching = source.ToLookup(pred);
    return (groupedByMatching[true], groupedByMatching[false]);
}

// Ex.
var numbers = new [] { 1, 2, 3, 4, 5, 6, 7, 8 };
var (numbersLessThanEqualFour, numbersMoreThanFour) = numbers.Fork(x => x <= 4);

Answer 3

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];