LDM Notes for May 20th, 2026

[333fred]

https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-05-20.md

Agenda

• Unions
  • Union pattern matching target for custom unions
  • Missing required APIs in custom union declarations
  • Direct value property matching and union rules

[bencyoung-Fignum]

I think it has to be the "try both" approach else I think it's even more confusing even for new union uses, and as you say can break simple code that's upgraded to be unions

[jnm2]

It will be breaking in many subtle ways even with the try-both proposal in its current form. For example, null checks on the container are implicitly extended to include the value when you add [Union] to the container type.

[TahirAhmadov]

Can't the union members interface be used to avoid this problem?

[CyrusNajmabadi]

How would the union members interface help here? Can you expand on this idea?

[TahirAhmadov]

Something like below, but I'm not 100% sure.

[Union]
struct MyResult<T> : MyResult<T>.IUnionMembers
{ 
  public interface IUnionMembers
  {
    public static Result<T> Create(T value) => new() { _value = value };
    public static Result<T> Create(Exception value) => new() { _value = value };

    public object? Value { get; }
    public MyResult<T> Additional { get; }
  }
  public bool IsOk => ...; public T OkValue => ...; 
  
  MyResult<T> IUnionMembers.Additional => this;
}

var result = new MyResult<int>(10);

if (result is { IsOk: true, OkValue: var v }) // at compile time, check if Additional property exists, and use that
    Use(v);