C# in Depth

116 CHAPTER 4 Saying nothing with nullable types4.2.1 Introducing NullableAs you can tell by its name, Nullable is a generic type. The type parameter T has thevalue type constraint on it. As I mentioned in section 3.3.1, this also means you can’tuse another nullable type as the argument—so Nullable is forbidden,for instance, even though Nullable is a value type in every other way. The typeof T for any particular nullable type is called the underlying type of that nullable type. Forexample, the underlying type of Nullable is int.The most important parts of Nullable are its properties, HasValue andValue. They do the obvious thing: Value represents the non-nullable value (the“real” one, if you will) when there is one, and throws an InvalidOperation-Exception if (conceptually) there is no real value. HasValue is simply a Booleanproperty indicating whether there’s a real value or whether the instance should beregarded as null. For now, I’ll talk about an “instance with a value” and an “instancewithout a value,” which mean instances where the HasValue property returns true orfalse, respectively.Now that we know what we want the properties to achieve, let’s see how to createan instance of the type. Nullable has two constructors: the default one (creatingan instance without a value) and one taking an instance of T as the value. Once aninstance has been constructed, it is immutable.NOTEValue types and mutability—A type is said to be immutable if it is designed sothat an instance can’t be changed after it’s been constructed. Immutabletypes often make life easier when it comes to topics such as multithreading,where it helps to know that nobody can be changing values in onethread while you’re reading them in a different one. However, immutabilityis also important for value types. As a general rule, value types shouldalmost always be immutable. If you need a way of basing one value onanother, follow the lead of DateTime and TimeSpan—provide methodsthat return a new value rather than modifying an existing one. That way,you avoid situations where you think you’re changing a variable but actuallyyou’re changing the value returned by a property or method, which is justa copy of the variable’s value. The compiler is usually smart enough towarn you about this, but it’s worth trying to avoid the situation in the firstplace. Very few value types in the framework are mutable, fortunately.Nullable introduces a single new method, GetValueOrDefault, which has twooverloads. Both return the value of the instance if there is one, or a default value otherwise.One overload doesn’t have any parameters (in which case the generic defaultvalue of the underlying type is used), and the other allows you to specify the defaultvalue to return if necessary.The other methods implemented by Nullable all override existing methods:GetHashCode, ToString, and Equals. GetHashCode returns 0 if the instance doesn’thave a value, or the result of calling GetHashCode on the value if there is one.ToString returns an empty string if there isn’t a value, or the result of callingLicensed to Rhona Hadida