It’s a truth widely acknowledged that null references in programming languages lead to faults. Newer languages, such as Apple’s Swift are controlling nulls, to avoid problems.
The .NET and C# design committees are looking at introducing non-nullable reference types in a future version of C#. This post outlines what I think are the requirements of (non-)nullable references in C#. A future post will describe a possible language design which fulfils them.
Existing languages, such as C# and Java, have a legacy of allowing any reference type to be null. This has several implications:
- Null is used by programmers to mark a ‘missing value’. However,
- There’s nothing in the type system to document whether a value can potentially be null or not, so:
- Programmers are required to document nullability in code comments, and enforce rules and conventions explicitly in code (which is a source of errors).
- The language runtime must check for a null reference before every object dereference (which is inefficient).
.NET 2.0 introduced the idea of nullable value types. This allows ints, doubles, bools, and other passed-by-value structures, to be marked as optional. C# uses the same terminology of ‘nullability’ to express this but ‘nullable’ value types are significantly different from nullable reference types:
- Nullable values have a different type from non-nullable values; an optional int has the type ‘
Nullable<int>’, which is usually abbreviated to ‘
int?’. Nullable value types are actually structures which contain a value type (or don’t, in the case of ‘
- The type system does not allow a null to be assigned to a non-nullable value type.
- Nullable numeric types can be used without checking for null. For example, ‘nullableA + nonNullableB’ has a well-defined result, and does not throw a
NullReferenceException. Nulls propagate through maths expressions, similarly to NULL values in SQL.
It would be undeniably useful to allow reference types which exclude null. Such types might be annotated with an ‘
!’ after the type name.
This could help guarantee that the software never faults with a
NullReferenceException. It also makes something explicit in the language which is currently implicit, thus removing a documentation and coding burden. For example, annotating a method parameter as non-nullable would mean that there is no need to document that nulls are disallowed, and no need for the method to test ‘
if (p == null) throw new ArgumentNullException("p");’ on its first line.
However, that’s only a partial improvement on what we have now. There is an opportunity to also introduce:
- Explicitly nullable reference types with the same semantics as nullable value types (i.e., you’d be able to declare a parameter as ‘string?’)
- Type uniformity between explicitly nullable value and reference types. That is to say: the type ‘
System.Nullable<T>’ would be able to accept any T regardless of whether T is a reference type or a value type.
Explicitly-nullable reference types may require more explanation: after all, they seem not to add anything new to the language. Actually they add several things:
- Safety: Explicitly-nullable reference types require explicit testing and dereferencing. ‘
if (n.HasValue) DoSomethingWith(n.Value)’. This removes ambiguity and one source of errors.
- Intent: Marking a parameter as explicitly nullable is a stronger indication of intent than an unannotated reference-type parameter, one which would default to allowing null.
- Uniformity: There is more uniformity between reference and value types (of which, more below).
- Potential: We raise the possibility of changing the default in future versions of the language, from reference types being nullable by default, to them being non-nullable by default.
What about type uniformity between nullable values and references? What does that add?
Uniformity simply makes writing type-generic code easier. It allows us to define, for example, an interface ‘
IParser<T>’ with method ‘
T? Parse(string str)’—regardless of whether T is a reference type or a value type.
In many cases the programmer should not have to care whether a type has value semantics or reference semantics (particularly if the type is immutable). By enforcing reference-value uniformity in nullable types, we remove a sometimes-artificial distinction between value types and reference types.
[Note that being able to use ‘
Nullable<T>’ for any type T implies that it would be possible to declare ‘
Nullable<Nullable<T>>’. This would rarely be required, but again, for reasons of uniformity, it would be a welcome addition to the language. At this point ‘
Nullable<T>’ is a misleading name, and ‘
Optional<T>’ would be a better name, but I suspect that that ship has already sailed.]
Interoperating with old .NET libraries
There is one other important requirement: interoperability.
It almost goes without saying that a new language feature must interoperate cleanly with legacy code, and not break source or binary compatibility. At a minimum:
- New, null-aware code must be able to pass parameters to non-null-aware methods in other libraries. This implies that there must be a way to pass (for example) a ‘
string!’ or a ‘
string?’ to a method which accepts an (implicitly-nullable) ‘string’. Similarly it must be possible to accept a non-null-aware reference result, and somehow massage it into a null-aware-reference form.
- Old, non-null-aware code should be able to call code written with the new null-aware style. This implies that old code which attempts to pass a ‘string’ to a method which accepts a ‘
string!’ may have an ‘
ArgumentNullException’ thrown by the runtime system if it attempts to pass in a
null value. Similarly, old code must be able to accept a null-aware reference type result and use it as it would a non-null-aware result.
- It would ideally be possible to gradually upgrade old code by adding ‘
?’ and ‘
!’ annotations to method parameters and results, without breaking source or binary compatibility—even overriding base class non-null-aware methods with null-aware ones.
These are my requirements. I’ll follow with another post soon, about a design for nullability in C# and .NET which fulfils these requirements.