C# in Depth

System.Nullable and System.Nullable119nullable = new Nullable();boxed = nullable;Console.WriteLine (boxed==null);nullable = (Nullable)boxed;Console.WriteLine(nullable.HasValue);Boxes a nullablewithout valueUnboxes tonullable variableThe output of listing 4.2 shows that the type of the boxed value is printed as System.Int32 (not System.Nullable). It then confirms that we can retrievethe value by unboxing to either just int or to Nullable. Finally, the output demonstrateswe can box from a nullable instance without a value to a null reference andsuccessfully unbox again to another value-less nullable instance. If we’d tried unboxingthe last value of boxed to a non-nullable int, the program would have blown up with aNullReferenceException.Now that we understand the behavior of boxing and unboxing, we can begin totackle the behavior of Nullable.Equals.4.2.3 Equality of Nullable instancesNullable overrides object.Equals(object) but doesn’t introduce any equalityoperators or provide an Equals(Nullable) method. Since the framework has suppliedthe basic building blocks, languages can add extra functionality on top, includingmaking existing operators work as we’d expect them to. We’ll see the details ofthat in section 4.3.3, but the basic equality as defined by the vanilla Equals methodfollows these rules for a call to first.Equals(second):■ If first has no value and second is null, they are equal.■ If first has no value and second isn’t null, they aren’t equal.■ If first has a value and second is null, they aren’t equal.■ Otherwise, they’re equal if first’s value is equal to second.Note that we don’t have to consider the case where second is another Nullablebecause the rules of boxing prohibit that situation. The type of second is object, so inorder to be a Nullable it would have to be boxed, and as we have just seen, boxinga nullable instance creates a box of the non-nullable type or returns a null reference.The rules are consistent with the rules of equality elsewhere in .NET, so you can usenullable instances as keys for dictionaries and any other situations where you needequality. Just don’t expect it to differentiate between a non-nullable instance and anullable instance with a value—it’s all been carefully set up so that those two cases aretreated the same way as each other.That covers the Nullable structure itself, but it has a shadowy partner: theNullable class.4.2.4 Support from the nongeneric Nullable classThe System.Nullable struct does almost everything you want it to. However, itreceives a little help from the System.Nullable class. This is a static class—it onlyLicensed to Rhona Hadida