C# in Depth

120 CHAPTER 4 Saying nothing with nullable typescontains static methods, and you can’t create an instance of it. 2 In fact, everything itdoes could have been done equally well by other types, and if Microsoft had seenwhere they were going right from the beginning, it might not have even existed—which would have saved a little confusion over what the two types are there for, asidefrom anything else. However, this accident of history has three methods to its name,and they’re still useful.The first two are comparison methods:public static int Compare(Nullable n1, Nullable n2)public static bool Equals(Nullable n1, Nullable n2)Compare uses Comparer.Default to compare the two underlying values (if theyexist), and Equals uses EqualityComparer.Default. In the face of instances withno values, the values returned from each method comply with the .NET conventionsof nulls comparing equal to each other and less than anything else.Both of these methods could quite happily be part of Nullable as static butnongeneric methods. The one small advantage of having them as generic methods ina nongeneric type is that generic type inference can be applied, so you’ll rarely needto explicitly specify the type parameter.The final method of System.Nullable isn’t generic—indeed, it absolutely couldn’tbe. Its signature is as follows:public static Type GetUnderlyingType (Type nullableType)If the parameter is a nullable type, the method returns its underlying type; otherwiseit returns null. The reason this couldn’t be a generic method is that if you knew theunderlying type to start with, you wouldn’t have to call it!We’ve now seen what the framework and the CLR provide to support nullabletypes—but C# 2 adds language features to make life a lot more pleasant.4.3 C# 2’s syntactic sugar for nullable typesThe examples so far have shown nullable types doing their job, but they’ve not beenparticularly pretty to look at. Admittedly it makes it obvious that you are using nullabletypes when you have to type Nullable around the name of the type you’re reallyinterested in, but it makes the nullability more prominent than the name of the typeitself, which is surely not a good idea.In addition, the very name “nullable” suggests that we should be able to assignnull to a variable of a nullable type, and we haven’t seen that—we’ve always used thedefault constructor of the type. In this section we’ll see how C# 2 deals with theseissues and others.Before we get into the details of what C# 2 provides as a language, there’s one definitionI can finally introduce. The null value of a nullable type is the value whereHasValue returns false—or an “instance without a value,” as I’ve referred to it in section4.2. I didn’t use it before because it’s specific to C#. The CLI specification2You’ll learn more about static classes in chapter 7.Licensed to Rhona Hadida