C# in Depth

130 CHAPTER 4 Saying nothing with nullable typesNote how the type of the result is DateTime rather than DateTime? because we’veused DateTime.Now as the second operand. We could shorten the whole thing to oneexpression:return (death ?? DateTime.Now)-birth;However, this is a bit more obscure—in particular, in the two-line version the name ofthe lastAlive variable helps the reader to see why we’re applying the null coalescingoperator. I hope you agree that the two-line version is simpler and more readable thaneither the original version using the if statement or the version using the normal conditionaloperator from C# 1. Of course, it relies on the reader understanding what thenull coalescing operator does. In my experience, this is one of the least well-knownaspects of C# 2, but it’s useful enough to make it worth trying to enlighten yourcoworkers rather than avoiding it.There are two further aspects that increase the operator’s usefulness, too. First, itdoesn’t just apply to nullable types—reference types can also be used; you just can’tuse a non-nullable value type for the first operand as that would be pointless. Also, it’sright associative, which means an expression of the form first ?? second ?? third isevaluated as first ?? (second ?? third)—and so it continues for more operands.You can have any number of expressions, and they’ll be evaluated in order, stoppingwith the first non-null result. If all of the expressions evaluate to null, the result will benull too.As a concrete example of this, suppose you have an online ordering system (andwho doesn’t these days?) with the concepts of a billing address, contact address, andshipping address. The business rules declare that any user must have a billing address,but the contact address is optional. The shipping address for a particular order is alsooptional, defaulting to the billing address. These “optional” addresses are easily representedas null references in the code. To work out who to contact in the case of aproblem with a shipment, the code in C# 1 might look something like this:Address contact = user.ContactAddress;if (contact==null){contact = order.ShippingAddress;if (contact==null){contact = user.BillingAddress;}}Using the conditional operator in this case is even more horrible. Using the null coalescingoperator, however, makes the code very straightforward:Address contact = user.ContactAddress ??order.ShippingAddress ??user.BillingAddress;If the business rules changed to use the shipping address by default instead of theuser’s contact address, the change here would be extremely obvious. It wouldn’t beLicensed to Rhona Hadida