C# in Depth

122 CHAPTER 4 Saying nothing with nullable typesGiven that the C# 2 specification defines the null value, it would be pretty odd if wecouldn’t use the null literal we’ve already got in the language in order to represent it.Fortunately we can, as our next section will show.4.3.2 Assigning and comparing with nullA very concise author could cover this whole section in a single sentence: “The C#compiler allows the use of null to represent the null value of a nullable type in bothcomparisons and assignments.” I prefer to show you what it means in real code, as wellas think about why the language has been given this feature.You may have felt a bit uncomfortable every time we’ve used the default constructorof Nullable. It achieves the desired behavior, but it doesn’t express the reasonwe want to do it—it doesn’t leave the right impression with the reader. We want togive the same sort of feeling that using null does with reference types. If it seems oddto you that I’ve talked about feelings in both this section and the last one, just thinkabout who writes code, and who reads it. Sure, the compiler has to understand thecode, and it couldn’t care less about the subtle nuances of style—but very few piecesof code used in production systems are written and then never read again. Anythingyou can do to get the reader into the mental process you were going through whenyou originally wrote the code is good—and using the familiar null literal helps toachieve that.With that in mind, we’re going to change the example we’re using from one thatjust shows syntax and behavior to one that gives an impression of how nullable typesmight be used. We’ll consider modeling a Person class where you need to know thename, date of birth, and date of death of a person. We’ll only keep track of peoplewho have definitely been born, but some of those people may still be alive—in whichcase our date of death is represented by null. Listing 4.4 shows some of the possiblecode. Although a real class would clearly have more operations available, we’re justlooking at the calculation of age for this example.Listing 4.4Part of a Person class including calculation of ageclass Person{DateTime birth;DateTime? death;string name;public TimeSpan Age{get{if (death==null){BChecksHasValuereturn DateTime.Now-birth;}else{return death.Value-birth;CUnwraps forcalculationLicensed to Rhona Hadida