C# in Depth

C# 2’s syntactic sugar for nullable types125So far, so relatively simple. Now let’s consider the operators, where things areslightly more tricky.OPERATORS INVOLVING NULLABLE TYPESC# allows the following operators to be overloaded:■ Unary: + ++ - -- ! ~ true false■ Binary: + - * / % & | ^ >■ Equality: 4 == !=■ Relational: < > =When these operators are overloaded for a non-nullable value type T, the nullable typeT? has the same operators, with slightly different operand and result types. These arecalled lifted operators whether they’re predefined operators like addition on numerictypes, or user-defined operators like adding a TimeSpan to a DateTime. There are a fewrestrictions as to when they apply:■ The true and false operators are never lifted. They’re incredibly rare in thefirst place, though, so it’s no great loss.■ Only operators with non-nullable value types for the operands are lifted.■ For the unary and binary operators (other than equality and relational operators),the return type has to be a non-nullable value type.■ For the equality and relational operators, the return type has to be bool.■ The & and | operators on bool? have separately defined behavior, which we’llsee in section 4.3.6.For all the operators, the operand types become their nullable equivalents. For theunary and binary operators, the return type also becomes nullable, and a null value isreturned if any of the operands is a null value. The equality and relational operatorskeep their non-nullable Boolean return types. For equality, two null values are consideredequal, and a null value and any non-null value are considered different, which isconsistent with the behavior we saw in section 4.2.3. The relational operators alwaysreturn false if either operand is a null value. When none of the operands is a null value,the operator of the non-nullable type is invoked in the obvious way.All these rules sound more complicated than they really are—for the most part,everything works as you probably expect it to. It’s easiest to see what happens with afew examples, and as int has so many predefined operators (and integers can be soeasily expressed), it’s the natural demonstration type. Table 4.1 shows a number ofexpressions, the lifted operator signature, and the result. It is assumed that there arevariables four, five, and nullInt, each with type int? and with the obvious values.Possibly the most surprising line of the table is the bottom one—that a null valueisn’t deemed “less than or equal to” another null value, even though they are deemed4The equality and relational operators are, of course, binary operators themselves, but we’ll see that theybehave slightly differently to the others, hence their separation here.Licensed to Rhona Hadida