C# in Depth

128 CHAPTER 4 Saying nothing with nullable typesTable 4.2Truth table for the logical operators AND, inclusive OR, exclusive OR, and logicalnegation, applied to the bool? type (continued)x y x & y x | y x ^ y !xnull true null true null nullnull false false null null nullnull null null null null nullFor those who find reasoning about rules easier to understand than looking up valuesin tables, the idea is that a null bool? value is in some senses a “maybe.” If you imaginethat each null entry in the input side of the table is a variable instead, then you willalways get a null value on the output side of the table if the result depends on thevalue of that variable. For instance, looking at the third line of the table, the expressiontrue & y will only be true if y is true, but the expression true | y will always betrue whatever the value of y is, so the nullable results are null and true, respectively.When considering the lifted operators and particularly how nullable logic works, thelanguage designers had two slightly contradictory sets of existing behavior—C# 1 nullreferences and SQL NULL values. In many cases these don’t conflict at all—C# 1 had noconcept of applying logical operators to null references, so there was no problem inusing the SQL-like results given earlier. The definitions we’ve seen may surprise someSQL developers, however, when it comes to comparisons. In standard SQL, the result ofcomparing two values (in terms of equality or greater than/less than) is always unknownif either value is NULL. The result in C# 2 is never null, and in particular two null valuesare considered to be equal to each other.NOTE Reminder: this is C# specific! It’s worth remembering that the lifted operatorsand conversions, along with the bool? logic described in this section,are all provided by the C# compiler and not by the CLR or the frameworkitself. If you use ildasm on code that evaluates any of these nullable operators,you’ll find that the compiler has created all the appropriate IL totest for null values and deal with them accordingly. This means that differentlanguages can behave differently on these matters—definitelysomething to look out for if you need to port code between different.NET-based languages.We now certainly know enough to use nullable types and predict how they’ll behave,but C# 2 has a sort of “bonus track” when it comes to syntax enhancements: the nullcoalescing operator.4.3.5 The null coalescing operatorAside from the ? modifier, all of the rest of the C# compiler’s tricks so far to do withnullable types have worked with the existing syntax. However, C# 2 introduces a newoperator that can occasionally make code shorter and sweeter. It’s called the null coalescingoperator and appears in code as ?? between its two operands. It’s a bit like theconditional operator but specially tweaked for nulls.Licensed to Rhona Hadida