Union Source Generator

Union Source Generator is a C# source generator that generates a union type for a set of types. The generated union type can hold any one of the specified types. Consuming the type can be done by exhaustive pattern matching.

The main component is one generic attribute, UnionAttribute, which is used to specify the types that the union can hold, on a struct:

[Union<Result<int>, NotFound>]
public readonly partial struct SimpleObj;

This will result in a generated SimpleObj type that can hold any of the specified types, but only one at a time. It also provides compile time checked exhaustive Switch and Match methods to handle the different types. Implicit conversions operators are generated as well as equality members.

SimpleObj simple = CreateSimple();
simple.Switch(
              r =>  Console.WriteLine($"Found: {r}"),
              _ => Console.WriteLine("not found"));
int result = simple.Match(r => r.Value * 2,
                          _ => -1);

SimpleObj CreateSimple() => new NotFound();

While the generator itself has to be a netstandard2.0 project, the generated code assumes C#12 / .NET 8 at this point.

This project is heavily influenced by the great OneOf library. All credit for the original concept to its authors!

Opinionated Naming Scheme

This library is opinionated as it will try to assign 'readable' names to the properties based on the specified types:

SimpleObj simple = new SimpleObj(new Result<int>(12));
bool notFound = simple.IsNotFound;
Result<int> result = simple.AsResultOfInt();

It even will try to detect collections and assign names like ListOfFoo or DictionaryOfStringAndLong.

The same is true for the lambda parameter names in the Match & Switch methods. For Switch they will get names like forString (or forNone) and for Match ones like withString (or withNone).

That can work great in many scenarios but will probably lead to bad naming in some cases - that's the trade-off I'm willing to accept.

Union Object Size

We try to be smart and use as little memory as possible for the union object.

• They are readonly structs
• Only those fields actually needed are generated
  • e.g. only a single reference field which is used for all reference types
  • if there are no reference types, no reference field is generated
  • if there are no value types, no value field is generated
• A single byte is used for storing the state so that they union object knows what it is

Plus padding for alignment done by the runtime.

Motivation

My main motivation was to finally learn more about writing source generators by creating one myself. I haven't found a lot of resources regarding generic marker attributes in combination with source generators, so I'm not sure my approach is optimal, but maybe it can serve as a starting point for others.

As a first project I wanted something with a small scope and I was always a little annoyed by the property names (T0, T1, ...) in the OneOf library (which they have to use due to the types being generic - even when using their source generator). So this is what I decided to tackle.

Quality

This is a two-day toy project without much testing (and no serious automated tests). I will probably use it in my own projects in the future to see how far I'll get and fix issues as they arise.

Feedback (and PRs 😉) to make the implementation more robust, efficient and generally better are welcome, of course!

Don't expect production grade reliability here!