Note: This article/proposal is an aggregation of several previous blog posts written over the past few years. The content is mostly the same, but has been edited for consistency and clarity.

Abstract

In the C# programming language, there are two kinds of data types: value types and reference types. Reference types can hold either pointers to objects, or the special value null, which is used to indicate a ‘missing’ reference. This feature of implicitly allowing nulls in reference types has some well-recognised problems and the .NET team are already exploring ways of enforcing non-nullable reference types; this paper explores one possible design. Continue reading →

C# Non-Nullable Types: Backwards Compatibility

In previous episodes I proposed a set of requirements for non-nullable reference types in C#/.NET, and a proposed design, including their interaction with generic types (polymorphic types).

There is also something to be said about backwards- and forwards- compatibility:

1. How can legacy code — using implicitly-nullable reference types — be migrated to using non-nullable references without causing disruption?
2. How should legacy code interact with APIs which accept or return non-nullable reference types?
3. What are the implications of generic code to backwards compatibility?

Continue reading →

In this series so far:

1. Some requirements for null-aware types in C#
2. A basic design
3. (This article) A wee digression
4. (Next time) How the design affects generic types
5. (Probably) Compatibility
6. (Probably) The Future

In this article we’ll digress about default values…

Continue reading →

In my previous post, I outlined a list of requirements for non-nullable (and explicitly-nullable) reference types in C#. In this post we’ll dive into some further design decisions. Subsequent posts will look at the impact on generic types, plus backward-compatibility and some corner cases.

Expanding the type system

We add two new main concepts to the type system:

1. non-null reference types T, denoted as ‘T!’, and
2. explicitly-nullable reference types, ‘T?’.

Continue reading →