/// <summary>
/// Actual code for SelectMany implementation. Instead of doing Cartesian Product, it
/// performs zipping.
/// </summary>
/// <typeparam name="TSource"></typeparam>
/// <typeparam name="TCollection"></typeparam>
/// <typeparam name="TResult"></typeparam>
/// <param name="source"></param>
/// <param name="collectionSelector"></param>
/// <param name="resultSelector"></param>
/// <returns></returns>
private static IEnumerable <TResult> SelectManyImpl <TSource, TCollection, TResult>(IZippable <TSource> source,
Func <TSource, int, IZippable <TCollection> > collectionSelector,
Func <TSource, TCollection, TResult> resultSelector)
{
int index = 0;
using (var e1 = source.GetEnumerator())
{
// a default(TSource) causes that using nested "from" in LINQ won't work. Example of what's not allowed:
// from entity in entities
// from element in entity.collection <- this won't work
// ....
using (var e2 = collectionSelector(default(TSource), index++).GetEnumerator())
{
while (true)
{
var isNext1 = e1.MoveNext();
var isNext2 = e2.MoveNext();
if (isNext1 && isNext2)
{
yield return(resultSelector(e1.Current, e2.Current));
}
else
{
yield break;
}
}
}
}
}
