C# in Depth

Extension methods in .NET 3.5267as a delegate. It’s very like the ConvertAll method in List, but operating on anyenumerable collection and using deferred execution to perform the projection only aseach element is requested.When I introduced anonymous types, I said they were useful with lambda expressionsand LINQ—well, here’s an example of the kind of thing you can do with them.We’ve currently got the odd numbers between 0 and 9 (in reverse order)—let’s createa type that encapsulates the square root of the number as well as the original number.Listing 10.9 shows both the projection and a slightly modified way of writing out theresults. I’ve adjusted the whitespace solely for the sake of space on the printed page.Listing 10.9Projection using a lambda expression and an anonymous typevar collection = Enumerable.Range(0, 10).Where(x => x%2 != 0).Reverse().Select(x => new { Original=x, SquareRoot=Math.Sqrt(x) } );foreach (var element in collection){Console.WriteLine("sqrt({0})={1}",element.Original,element.SquareRoot);}This time the type of collection isn’t IEnumerable—it’s IEnumerable,where Something is the anonymous type created by the compiler. We can’texplicitly type the collection variable except as either the nongeneric IEnumerabletype or object. Implicit typing is what allows us to use the Original and SquareRootproperties when writing out the results. The output of listing 10.9 is as follows:sqrt(9)=3sqrt(7)=2.64575131106459sqrt(5)=2.23606797749979sqrt(3)=1.73205080756888sqrt(1)=1Of course, a Select method doesn’t have to use an anonymous type at all—we couldhave selected just the square root of the number, discarding the original. In that casethe result would have been IEnumerable. Alternatively, we could have manuallywritten a type to encapsulate an integer and its square root—it was just easiest touse an anonymous type in this case.Let’s look at one last method to round off our coverage of Enumerable for themoment: OrderBy.10.3.4 Sorting using the OrderBy methodSorting data is a common requirement when processing data, and in LINQ this is usuallyperformed using the OrderBy or OrderByDescending methods, sometimes followedby ThenBy or ThenByDescending if you need to sort by more than one propertyof the data. This ability to sort on multiple properties has always been available theLicensed to Rhona Hadida