C# in Depth

C# 2’s syntactic sugar for nullable types127Now we’re able to answer the question at the end of the previous section—why weused death.Value-birth in listing 4.4 instead of just death-birth. Applying the previousrules, we could have used the latter expression, but the result would have been aTimeSpan? instead of a TimeSpan. This would have left us with the options of casting theresult to TimeSpan, using its Value property, or changing the Age property to return aTimeSpan?—which just pushes the issue onto the caller. It’s still a bit ugly, but we’ll seea nicer implementation of the Age property in section 4.3.5.In the list of restrictions regarding operator lifting, I mentioned that bool? worksslightly differently than the other types. Our next section explains this and pulls backthe lens to see the bigger picture of why all these operators work the way they do.4.3.4 Nullable logicI vividly remember my early electronics lessons at school. They always seemed torevolve around either working out the voltage across different parts of a circuit usingthe V=IR formula, or applying truth tables—the reference charts for explaining the differencebetween NAND gates and NOR gates and so on. The idea is simple—a truthtable maps out every possible combination of inputs into whatever piece of logicyou’re interested in and tells you the output.The truth tables we drew for simple, two-input logic gates always had four rows—each of the two inputs had two possible values, which means there were four possiblecombinations. Boolean logic is simple like that—but what should happen when you’vegot a tristate logical type? Well, bool? is just such a type—the value can be true,false, or null. That means that our truth tables now have to have nine rows for ourbinary operators, as there are nine combinations. The specification only highlightsthe logical AND and inclusive OR operators (& and |, respectively) because the otheroperators—unary logical negation (!) and exclusive OR (^)—follow the same rules asother lifted operators. There are no conditional logical operators (the short-circuiting&& and || operators) defined for bool?, which makes life simpler.For the sake of completeness, table 4.2 gives the truth tables for all four valid bool?operators.Table 4.2Truth table for the logical operators AND, inclusive OR, exclusive OR, and logicalnegation, applied to the bool? typex y x & y x | y x ^ y !xtrue true true true false falsetrue false false true true falsetrue null null true null falsefalse true false true true truefalse false false false false truefalse null false null null trueLicensed to Rhona Hadida