C# in Depth

118 CHAPTER 4 Saying nothing with nullable typesinstance. Next, we create an instance that doesn’t have a value, and use the same membersin the same order, just omitting the Value property and the explicit conversion toint since these would throw exceptions. The output of listing 4.1 is as follows:Instance with value:HasValue: TrueValue: 5Explicit conversion: 5GetValueOrDefault(): 5GetValueOrDefault(10): 5ToString(): "5"GetHashCode(): 5Instance without value:HasValue: FalseGetValueOrDefault(): 0GetValueOrDefault(10): 10ToString(): ""GetHashCode(): 0So far, you could probably have predicted all of the results just by looking at the membersprovided by Nullable. When it comes to boxing and unboxing, however,there’s special behavior to make nullable types behave how we’d really like them tobehave, rather than how they’d behave if we slavishly followed the normal boxing rules.4.2.2 Boxing and unboxingIt’s important to remember that Nullable is a struct—a value type. This means thatif you want to convert it to a reference type (object is the most obvious example), you’llneed to box it. It is only with respect to boxing and unboxing that the CLR itself has anyspecial behavior regarding nullable types—the rest is “standard” generics, conversions,method calls, and so forth. In fact, the behavior was only changed shortly before therelease of .NET 2.0, as the result of community requests.An instance of Nullable is boxed to either a null reference (if it doesn’t have avalue) or a boxed value of T (if it does). You can unbox from a boxed value either toits normal type or to the corresponding nullable type. Unboxing a null reference willthrow a NullReferenceException if you unbox to the normal type, but will unbox toan instance without a value if you unbox to the appropriate nullable type. This behavioris shown in listing 4.2.Listing 4.2Boxing and unboxing behavior of nullable typesNullable nullable = 5;object boxed = nullable;Console.WriteLine(boxed.GetType());Boxes a nullablewith valueint normal = (int)boxed;Console.WriteLine(normal);nullable = (Nullable)boxed;Console.WriteLine(nullable);Unboxes to nonnullablevariableUnboxes tonullable variableLicensed to Rhona Hadida