C# in Depth

306 CHAPTER 11 Query expressions and LINQ to ObjectsLeft=3; Right=11Left=3; Right=12Left=3; Right=13Left=4; Right=11Left=4; Right=12Left=4; Right=13Left=4; Right=14The method the compiler calls to generate this sequence is SelectMany. It takes a singleinput sequence (the left sequence in our terminology), a delegate to generateanother sequence from any element of the left sequence, and a delegate to generate aresult element given an element of each of the sequences. Here’s the signature of themethod, again written as if it were an instance method on IEnumerable:public IEnumerable SelectMany (Func collectionSelector,Func resultSelector)As with the other joins, if the part of the query expression following the join is a selectclause, that projection is used as the final argument; otherwise, a transparent identifieris introduced to make both the left and right sequences’ range variables available.Just to make this all a bit more concrete, here’s the query expression of listing 11.16,as the translated source code:Enumerable.Range(1, 4).SelectMany (left => Enumerable.Range(11, left),(left, right) => new {Left=left,Right=right})One interesting feature of SelectMany is that the execution is completely streamed—it only needs to process one element of each sequence at a time, because it uses afreshly generated right sequence for each different element of the left sequence.Compare this with inner joins and group joins: they both load the right sequencecompletely before starting to return any results. You should bear in mind theexpected size of sequence, and how expensive it might be to evaluate it multipletimes, when considering which type of join to use and which sequence to use as theleft and which as the right.This behavior of flattening a sequence of sequences, one produced from each elementin an original sequence, can be very useful. Consider a situation where youmight want to process a lot of log files, a line at a time. We can process a seamlesssequence of lines, with barely any work. The following pseudo-code is filled in morethoroughly in the downloadable source code, but the overall meaning and usefulnessshould be clear:var query = from file in Directory.GetFiles(logDirectory, "*.log")from line in new FileLineReader(file)let entry = new LogEntry(line)where entry.Type == EntryType.Errorselect entry;Licensed to Rhona Hadida