C# in Depth

124 CHAPTER 4 Saying nothing with nullable types4.3.3 Nullable conversions and operatorsWe’ve seen that we can compare instances of nullable types with null, but there areother comparisons that can be made and other operators that can be used in somecases. Likewise we’ve seen wrapping and unwrapping, but other conversions can beused with some types. This section explains what’s available. I’m afraid it’s pretty muchimpossible to make this kind of topic genuinely exciting, but carefully designed featureslike these are what make C# a pleasant language to work with in the long run.Don’t worry if not all of it sinks in the first time: just remember that the details are hereif you need to reference them in the middle of a coding session.The “executive summary” is that if there is an operator or conversion available ona non-nullable value type, and that operator or conversion only involves other nonnullablevalue types, then the nullable value type also has the same operator or conversionavailable, usually converting the non-nullable value types into their nullableequivalents. To give a more concrete example, there’s an implicit conversion from intto long, and that means there’s also an implicit conversion from int? to long? thatbehaves in the obvious manner.Unfortunately, although that broad description gives the right general idea, theexact rules are slightly more complicated. Each one is simple, but there are quite a fewof them. It’s worth knowing about them because otherwise you may well end up staringat a compiler error or warning for a while, wondering why it believes you’re tryingto make a conversion that you never intended in the first place. We’ll start with theconversions, and then look at operators.CONVERSIONS INVOLVING NULLABLE TYPESFor completeness, let’s start with the conversions we already know about:■ An implicit conversion from the null literal to T?■ An implicit conversion from T to T?■An explicit conversion from T? to TNow consider the predefined and user-defined conversions available on types. Forinstance, there is a predefined conversion from int to long. For any conversion likethis, from one non-nullable value type (S) to another (T), the following conversionsare also available:■ S? to T? (explicit or implicit depending on original conversion)■ S to T? (explicit or implicit depending on original conversion)■ S? to T (always explicit)To carry our example forward, this means that you can convert implicitly from int? tolong? and from int to long? as well as explicitly from long? to int. The conversionsbehave in the natural way, with null values of S? converting to null values of T?, andnon-null values using the original conversion. As before, the explicit conversion fromS? to T will throw an InvalidOperationException when converting from a null valueof S?. For user-defined conversions, these extra conversions involving nullable typesare known as lifted conversions.Licensed to Rhona Hadida