C# 14 (Preview) 技術レポート — Before / After で分かる新機能まとめ

<!-- .csproj 抜粋 -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <LangVersion>preview</LangVersion>
  </PropertyGroup>
</Project>

public static class EnumerableExt
{
    public static bool IsEmpty<T>(this IEnumerable<T> src) => !src.Any();

    // 静的っぽいユーティリティは別メソッドで……
    public static IEnumerable<T> Identity<T>() => Enumerable.Empty<T>();
}

public static class EnumerableExt
{
    // インスタンス拡張
    extension<T>(IEnumerable<T> src)
    {
        public bool IsEmpty => !src.Any();
        public T this[int index] => src.Skip(index).First();
    }

    // 静的拡張
    extension<T>(IEnumerable<T>)
    {
        public static IEnumerable<T> Identity => Enumerable.Empty<T>();
        public static IEnumerable<T> Combine(IEnumerable<T> a, IEnumerable<T> b)
            => a.Concat(b);
    }
}

var seq = new[] {1, 2, 3};
bool empty = seq.IsEmpty;                 // インスタンス拡張
var id    = IEnumerable<int>.Identity;    // 静的拡張

private string _message;
public string Message
{
    get => _message;
    set => _message = value ?? throw new ArgumentNullException(nameof(value));
}

public string Message
{
    get;
    set => field = value ?? throw new ArgumentNullException(nameof(value));
}

if (customer != null)
{
    customer.Order = CreateOrder();
}

customer?.Order = CreateOrder(); // null なら右辺も評価されない

List<string>? log = null;
log?.Add("OK");               // 何も起きない
(log ??= []).Add("init");     // null のときだけ初期化

Console.WriteLine(typeof(Dictionary<,>).Name); // "Dictionary`2" などとなりがち

Console.WriteLine(nameof(Dictionary<,>)); // "Dictionary"

void Process(ReadOnlySpan<byte> data) { /* ... */ }

byte[] bytes = File.ReadAllBytes(path);
Process(bytes.AsSpan());          // 明示的な呼び出しが必要
ReadOnlySpan<byte> ro = bytes.AsSpan();

void Process(ReadOnlySpan<byte> data) { /* ... */ }

byte[] bytes = File.ReadAllBytes(path);
Process(bytes);                   // 暗黙変換で OK

Span<byte> span = bytes;
ReadOnlySpan<byte> ro = span;     // これも暗黙変換

delegate bool TryParseInt(string s, out int result);
TryParseInt parse = (string text, out int result) => int.TryParse(text, out result);

delegate bool TryParse<T>(string s, out T result);
TryParse<int> parse = (text, out result) => int.TryParse(text, out result);

public partial class User
{
    public User(string name)  // 全実装を1か所にまとめるしかない
    {
        Name = name;
    }

    public event Action<string>? Changed; // 同上
    public string Name { get; }
}

public partial class User
{
    public partial User(string name);
    public partial event Action<string> Changed;
}

// 別ファイルなどに実装
public partial class User
{
    public string Name { get; }
    public partial User(string name) => Name = name;

    public partial event Action<string> Changed
    {
        add { /* 登録処理 */ }
        remove { /* 解除処理 */ }
    }
}

public readonly struct Money(decimal amount)
{
    public decimal Amount { get; } = amount;
    public static Money operator +(Money a, Money b) => new(a.Amount + b.Amount);
    // += は C# が + を使って解決するが、明示的には定義不可
}

public readonly struct Money(decimal amount, string currency)
{
    public decimal Amount { get; } = amount;
    public string Currency { get; } = currency;

    public static Money operator +(Money l, Money r)
        => l.Currency == r.Currency
            ? new Money(l.Amount + r.Amount, l.Currency)
            : throw new InvalidOperationException();

    public static Money operator +=(Money l, Money r) => l + r;
}

// ++/-- はユーザー定義不可。Increment() などで代替

public struct Counter(int value)
{
    public int Value { get; private set; } = value;
    public static Counter operator ++(Counter c) => new(++c.Value);
    public static Counter operator --(Counter c) => new(--c.Value);
}