Nullable reference types in C#, some requirements

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:

  1. 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 ‘null’).
  2. The type system does not allow a null to be assigned to a non-nullable value type.
  3. 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 (unannotated) ‘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.

Death meets Terry Pratchett

When I was young, before your World Wide Webs and your Twitters, we used to have ‘newsgroups’—kind of Internet discussion forums. They were named like domain names, with dots: rec.arts.cycling, say, or

I subscribed to the Muppets one, and a few others, including and Douglas Adams (dns) used to pop in and say hello on his one occasionally (which was always promptly followed by a raft of newbies accusing the great man of being an imposter).

Terry Pratchett (pterry) was almost a permanent fixture of his. Amongst the silly chat, there were some in-jokes which occasionally made it into his books. He clearly loved his fans, and they loved him. You can’t fake that: he was generous of spirit. His books are influential; his books are imbued with a love of humanity reflecting his own, and his influence spread far beyond his books.

Plus, given the amount of good press he gave to Death, you’ve got to imagine that Death will be kind to him.


Wrapping exceptions for fun and profit

One for the programmers. (If you’re not one, look away now.)

Whenever I work on a nontrivial C# program, I usually end up writing the extension method shown at the bottom of this post. It lets me provide informative error messages when an exception bubbles up to the user, and promotes good error-handling generally. This is me sharing it with you, so you can enjoy it too.

Given an Exception, possibly containing inner exceptions, ‘ToFriendlyString()’ produces an ‘explanation’ from the chain of Exception.Messages: “This failed, because that failed, because the other failed.” Continue reading

House of Lords in Living-Up-To-Stereotype Shocker

House of Lords dining roomRecent news is that the House of Lords refused to share catering services with the House of Commons, for fear that the quality of their free food would decline—and particularly that they’d be served a lesser vintage of champagne. (Champagne wars in the Lords as peers say no to a cheaper vintage, The Observer, 7 Dec 2014)

They’ve spent about £2.6m of our tax money on champers in the last 4 years.

Meanwhile, the Red Cross is delivering emergency food in the UK for the first time since WWII, food banks are on the increase, the government has introduced swingeing austerity measures & the number of homeless families has increased by 18% in the last 4 years.

It seems immoral that we should be paying for free bubbly for the rich and privileged, when we seemly cannot afford to help the poorest out of destitution. There’s a petition to protest it on

The lords can have a glass of free champagne when the last food bank closes. I’ll buy the round.


When I was a child, I thought America was a place of racial harmony. At my school in Birmingham, (England, not Alabama) there was the average amount of racism, but on American TV shows, the U.S. seemed to my 8-year-old-eyes like a post-racial utopia: cops were always one-white, one-black; judges were invariably black women; silver-haired white guys adopted sassy black kids.

Of course, my 8-year-old self knew nothing. Continue reading