Clarify usage of IUnionMembers interface

[BarionLP]

Summary

clarify that all union members must be part of the interface
so far the paragraph mostly talks about factory methods.

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docs/csharp/language-reference/builtin-types/union.md	docs/csharp/language-reference/builtin-types/union

[BillWagner]

Hi @BarionLP

This is a good start. I'd like to see the example updated to include the additional members. So, instead of:

[System.Runtime.CompilerServices.Union]
public record class Outcome<T> : Outcome<T>.IUnionMembers
{
    private readonly object? _value;

    private Outcome(object? value) => _value = value;

    public interface IUnionMembers
    {
        static Outcome<T> Create(T? value) => new(value);
        static Outcome<T> Create(Exception? value) => new(value);
        object? Value { get; }
    }

    object? IUnionMembers.Value => _value;
}

The sample should include:

[System.Runtime.CompilerServices.Union]
public record class Outcome<T> : Outcome<T>.IUnionMembers
{
    private readonly object? _value;

    private Outcome(object? value) => _value = value;

    public interface IUnionMembers
    {
        static Outcome<T> Create(T? value) => new(value);
        static Outcome<T> Create(Exception? value) => new(value);
        object? Value { get; }

        bool TryGetValue(out T value);
        bool TryGetValue(out Exception value);
    }

    object? IUnionMembers.Value => _value;
}

Can you add that as well?

[BillWagner]

This probably shouldn't be a note. Instead, I'd flow it into the regular text, based on Alexsey's comments on the runtime issue:

Suggested change

	> [!NOTE]
	> If an `IUnionMembers` interface is used, all union members must be defined on that interface.
	A union type can delegate its union members to a nested `IUnionMembers` interface. When this interface is present, the compiler looks for `Create` factory methods instead of constructors:
	A union type can delegate its union members to a nested `IUnionMembers` interface. When this interface is present, the `union` type behaves as a *union member provider*. The compiler generates code to call the `IUnionMembers` interface. It won't generate calls to members declared on the union type that aren't members of the nested `IUnionMembers` interface. As the following example shows, that means you must add the necessary factory methods and the appropriate `TryGetValue` methods for all case types: