Question

I need to intersect 2 arrays (List<T>) by keeping just the distinct elements and mix the elements of one array with the other like this:

    A1 B1 C1 D1
    W2 X2 Y2 Z2 
 => A1 W2 B1 X2 C1 Y2 D1 Z2


    A1 B1 A1 D1
    W2 X2 Y2 Z2 
 => A1 W2 B1 X2 Y2 D1 Z2 // repeating A1


    A1 B1 A1 D1
    Z2 X2 Y2 Z2 
 => A1 Z2 B1 X2 Y2 D1  // repeating A1, Z2

    A1 B1 C1 D1 E1
    Z2 X2 Y2 
 => A1 Z2 B1 X2 C1 Y2 D1 E1

What is the optimal LINQ query to do it?

Answer 1

An interleave extension method (replaces Zip/SelectMany in Jon Skeets version). This may be a little faster and it includes the remaining of the longest list if the lists aren’t of equal length:

public static IEnumerable<T> Interleave<T>(this IEnumerable<T> first, IEnumerable<T> second)
{
    var e0 = first.GetEnumerator();
    var e1 = second.GetEnumerator();

    bool e0HasValue = false;
    bool e1HasValue = false;

    // Could be made a little faster by not checking both when one is empty
    while ((e0HasValue = e0.MoveNext()) | (e1HasValue = e1.MoveNext()))
    {
        if (e0HasValue)
            yield return e0.Current;

        if (e1HasValue)
            yield return e1.Current;
    }
}

And if you want distinct items, use:

var result = firstList.Interleave(secondList).Distinct();

This answer, of cause, doesn’t solve the uncertainty about Distinct as described in comments to Jon Skeet´s answer. But it could be combined with Jon Skeet´s order preserving extension method.