Programming with C#

I need a report which shows a comparison of the sales of each service with the previous year. Some services might be new and they don't appear in the previous year, others might be put off and so there are no sales for them in the current year. There can be services sold in both periods.

There are 4 overloads of the SelectMany method . Two of them project the index of each source element, for example: string[] sentenceSequences = new string[] {"The quick brown", "fox jumped over","the lazy dog."}; sentenceSequences.SelectMany( // index - the position of the sequence in the sentenceSequences array (sequence, index) => // check if index is on odd position and if so call ToUpper() // ... put the sequence in other array, as the result index % 2 == 0 ? new [] {sequence.ToUpper()} : new [] { sequence } ) .Dump(); But there is no overload for projecting the index of the result also (the index of the element in the result sequence). So here an implementation of it: public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, int, TResult> resul