C# in Depth

Introducing LINQ279Figure 11.1 shows this query expressiongraphically, breaking it down intoits individual steps. I’ve included anumber of similar figures in this chapter,but unfortunately for complicatedqueries there is simply not enoughroom on the printed page to show asmuch data as we might like. Moredetailed diagrams are available on thebook’s website.Each arrow represents a sequence—the description is on the left side, andsome sample data is on the right. Eachbox is a transformation from our queryexpression. Initially, we have the wholefamily (as Person objects); then afterfiltering, the sequence only containsadults (again, as Person objects); andthe final result has the names of thoseadults as strings. Each step simply takesone sequence and applies an operationAll "Person" objectsin "people"All "Person" objects withan age of at least 18Names of people withan age of at least 18from person in peoplewhere person.Age >= 18select person.Name(Result of query)Name="Holly", Age=31Name="Tom", Age=4Name="Jon", Age=31Name="William", Age=1Name="Robin", Age=1Name="Holly", Age=31Name="Jon", Age=31"Holly""Jon"Figure 11.1 A simple query expression broken downinto the sequences and transformations involvedto produce a new sequence. The result isn’t the strings “Holly” and “Jon” —instead, it’san IEnumerable, which, when asked for its elements one by one, will first yield“Holly” and then “Jon.”This example was straightforward to start with, but we’ll apply the same techniquelater to more complicated query expressions in order to understand them more easily.Some advanced operations involve more than one sequence as input, but it’s still a lotless to worry about than trying to understand the whole query in one go.So, why are sequences so important? They’re the basis for a streaming model fordata handling—one that allows us to process data only when we need to.DEFERRED EXECUTION AND STREAMINGWhen the query expression shown in figure 11.1 is created, no data is processed. Theoriginal list of people isn’t accessed at all. Instead, a representation of the query is builtup in memory. Delegate instances are used to represent the predicate testing for adulthoodand the conversion from a person to that person’s name. It’s only when the resultingIEnumerable is asked for its first element that the wheels start turning.This aspect of LINQ is called deferred execution. When the first element of the resultis requested, the Select transformation asks the Where transformation for its first element.The Where transformation asks the list for its first element, checks whether thepredicate matches (which it does in this case), and returns that element back toSelect. That in turn extracts the name and returns it as the result.That’s all a bit of a mouthful, but a sequence diagram makes it all much clearer.I’m going to collapse the calls to MoveNext and Current to a single fetch operation: itLicensed to Rhona Hadida