LDM Notes for February 11th, 2026

[333fred]

https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-02-11.md

Agenda

• Union patterns update

[hez2010]

As for the type of Value in [Union] and IUnionMembers, can it be allowed to use a more specific type instead of object or object??

In current proposal the Value property must be object or object?, but I do see the use cases where Value may be a more specific reference type like TBase? if all case types are implicitly convertible to TBase.

For example, property / list pattern matching then can work on such a union directly:

[Union]
struct AnyStream
{
    private readonly Stream? _value;
    public AnyStream(FileStream s) => _value = s;
    public AnyStream(MemoryStream s) => _value = s;
    public Stream? Value => _value;
}

if (u is { CanRead: true }) { } // OK

[Union]
struct IntSeq
{
    private readonly IReadOnlyList<int>? _value;
    public IntSeq(int[] a) => _value = a;
    public IntSeq(List<int> l) => _value = l;
    public IReadOnlyList<int>? Value => _value;
}

if (u is [1, .., 3]) { } // OK

[CyrusNajmabadi]

can it be allowed to use a more specific type instead of object or object??

We are designing things such that that is a capability that is possible. Whether or not it is immediately available in the guest version is tbd.

but I do see the use cases where Value may be a more specific reference type like TBase? if all case types are implicitly convertible to TBase.

What is the use case. As far as the language is concerned. The case types entirely determine everything.

It might help a tiny amount with codegen in some cases. But that's about it. It won't meaningfully change the semantics.

[jnm2]

guest->first