C# in Depth

58 CHAPTER 2 Core foundations: building on C# 1Extension methods also let you appear to add methods with implementations tointerfaces—and indeed that’s what LINQ relies on heavily, allowing calls to all kinds ofmethods on IEnumerable that have never previously existed.Here’s the quick-view list of these features, along with which version of C# they’reintroduced in:■ Generics—C# 2■ Limited delegate covariance/contravariance—C# 2■ Anonymous types—C# 3■ Implicit typing—C# 3■ Extension methods—C# 3After that fairly diverse set of features on the type system in general, let’s look at thefeatures added to one very specific part of typing in .NET—value types.2.4.3 Features related to value typesThere are only two features to talk about here, and C# 2 introduces them both. Thefirst goes back to generics yet again, and in particular collections. One common complaintabout using value types in collections with .NET 1.1 was that due to all of the“general purpose” APIs being specified in terms of the object type, every operationthat added a struct value to a collection would involve boxing it, and when retrieving ityou’d have to unbox it. While boxing is pretty cheap on a “per call” basis, it can causea significant performance hit when it’s used every time with frequently accessed collections.It also takes more memory than it needs to, due to the per-object overhead.Generics fix both the speed and memory deficiencies by using the real type involvedrather than just a general-purpose object. As an example, it would have been madnessto read a file and store each byte as an element in an ArrayList in .NET 1.1—but in.NET 2.0 it wouldn’t be particularly crazy to do the same with a List.The second feature addresses another common cause of complaint, particularlywhen talking to databases—the fact that you can’t assign null to a value type variable.There’s no such concept as an int value of null, for instance, even though a databaseinteger field may well be nullable. At that point it can be hard to model the databasetable within a statically typed class without a bit of ugliness of some form or another.Nullable types are part of .NET 2.0, and C# 2 includes extra syntax to make them easyto use. Listing 2.7 gives a brief example of this.Listing 2.7Demonstration of a variety of nullable type featuresint? x = null; Declares and sets nullable variablex = 5;if (x != null) Tests for presence of “real” value{int y = x.Value; Obtains “real” valueConsole.WriteLine (y);}int z = x ?? 10; Uses null-coalescing operatorLicensed to Rhona Hadida