union.cs

using System;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;

TestStandardUnion();
TestCustomUnion1();
TestCustomUnion2();
TestCustomUnion3();

void TestStandardUnion()
{
    StandardUnion u = 1;

    var underlyingType = u switch
    {
        int => "int",
        string => "string",
        _ => "unknown"
    };

    Console.WriteLine($"union is {underlyingType}.");
}

void TestCustomUnion1()
{
    CustomUnion1 u = 1;

    var underlyingType = u switch
    {
        int => "int",
        string => "string",
        _ => "unknown"
    };

    Console.WriteLine($"union is {underlyingType}.");
}

void TestCustomUnion2()
{
    CustomUnion2 u = 1;

    var underlyingType = u switch
    {
        int => "int",
        string => "string",
        _ => "unknown"
    };

    Console.WriteLine($"union is {underlyingType}.");
}

void TestCustomUnion3()
{
    CustomUnion3 u = 1;

    var underlyingType = u switch
    {
        int => "int",
        string => "string",
        _ => "unknown"
    };

    Console.WriteLine($"union is {underlyingType}.");
}

// boxing する
union StandardUnion(int, string);

// boxing する
// public な 1 引数コンストラクタは union constructor になる
[Union]
readonly struct CustomUnion1 : IUnion
{
    public CustomUnion1(int value) => Value = value;
    public CustomUnion1(string value) => Value = value;

    public object? Value { get; }
}

// boxing しない
// public な 1 引数コンストラクタは union constructor になる
[Union]
readonly struct CustomUnion2 : IUnion
{
    private enum ValueKind
    {
        None,
        Int,
        String
    }

    private readonly ValueKind _kind;
    private readonly int _intValue;
    private readonly string? _stringValue;

    public CustomUnion2(int value)
    {
        _kind = ValueKind.Int;
        _intValue = value;
    }

    public CustomUnion2(string value)
    {
        _kind = ValueKind.String;
        _stringValue = value;
    }

    public bool TryGetValue(out int value)
    {
        if (_kind != ValueKind.Int)
        {
            value = 0;
            return false;
        }

        value = _intValue;
        return true;
    }

    public bool TryGetValue([MaybeNullWhen(false)] out string value)
    {
        if (_kind != ValueKind.String)
        {
            value = null;
            return false;
        }

        value = _stringValue!;
        return true;
    }

    public object? Value =>
        _kind switch
        {
            ValueKind.None => null,
            ValueKind.Int => _intValue,
            ValueKind.String => _stringValue,
            _ => throw new UnreachableException()
        };
}

// boxing しない
// IUnionMembers.Create に定義された型変換のみが union constructor になる
[Union]
readonly struct CustomUnion3 : IUnion, CustomUnion3.IUnionMembers
{
    public interface IUnionMembers
    {
        static abstract CustomUnion3 Create(int value);
        static abstract CustomUnion3 Create(string value);

        bool TryGetValue(out int value);
        bool TryGetValue([MaybeNullWhen(false)] out string value);

        object? Value { get; }
    }

    private enum ValueKind
    {
        None,
        Int,
        String
    }

    private readonly ValueKind _kind;
    private readonly int _intValue;
    private readonly string? _stringValue;

    public static CustomUnion3 Create(int value) => new(value);
    public static CustomUnion3 Create(string value) => new(value);

    // これは union constructor ではない
    // CustomUnion3 u = new CustomUnion3(1.0); として呼び出すことは可能
    // CustomUnion3 u = 1.0; は不可能
    public CustomUnion3(double value)
    {
    }

    private CustomUnion3(int value)
    {
        _kind = ValueKind.Int;
        _intValue = value;
    }

    private CustomUnion3(string value)
    {
        _kind = ValueKind.String;
        _stringValue = value;
    }

    public bool TryGetValue(out int value)
    {
        if (_kind != ValueKind.Int)
        {
            value = 0;
            return false;
        }

        value = _intValue;
        return true;
    }

    public bool TryGetValue([MaybeNullWhen(false)] out string value)
    {
        if (_kind != ValueKind.String)
        {
            value = null;
            return false;
        }

        value = _stringValue!;
        return true;
    }

    public object? Value => _kind switch
    {
        ValueKind.None => null,
        ValueKind.Int => _intValue,
        ValueKind.String => _stringValue,
        _ => throw new UnreachableException()
    };
}