【C#】C# 14 逆引き — field・拡張メンバー・null条件代入

// 1. バッキングフィールドを書かずに検証する
public string Name { get; set => field = value.Trim(); }

// 2. 既存の型にプロパティや演算子を後から足す
extension(Point p)
{
    public double Length => Math.Sqrt((p.X * p.X) + (p.Y * p.Y));
}

// 3. null かもしれない相手に安全に代入する
maybe?.Level = 9;

// 4. ジェネリック型の名前を nameof で取る
nameof(List<>)                       // "List"

// 5. 配列を Span 引数にそのまま渡す
Sum(numbers);                        // int[] -> ReadOnlySpan<int>

// 6. 型を書かずに out 付きラムダを書く
TryParse<int> parse = (text, out value) => int.TryParse(text, out value);

// ---- 1. field キーワード ----
public class Member
{
    public Member()
    {
        Name = "名無し";
    }

    public string Name
    {
        get;
        set => field = string.IsNullOrWhiteSpace(value) ? "名無し" : value.Trim();
    }

    public int Level
    {
        get => field;
        set
        {
            if (value < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(value));
            }

            field = value;
        }
    }
}

var m = new Member();
m.Name = "   ";
m.Level = 3;
Console.WriteLine($"1: [{m.Name}] {m.Level}");

// 実行結果
// 1: [名無し] 3

// ---- 2. 拡張メンバー ----
public readonly record struct Point(double X, double Y);

public static class PointExtensions
{
    extension(Point p)
    {
        public double Length => Math.Sqrt((p.X * p.X) + (p.Y * p.Y));
        public static Point Origin => new Point(0, 0);
        public static Point operator +(Point a, Point b) => new Point(a.X + b.X, a.Y + b.Y);
    }

    extension<T>(IEnumerable<T> source)
    {
        public bool IsEmpty => !source.Any();
    }
}

Point a = new Point(3, 4);
Point b = new Point(1, 2);
Console.WriteLine($"2: {a.Length} {Point.Origin} {a + b} {new[] { 1 }.IsEmpty}");

// 実行結果
// 2: 5 Point { X = 0, Y = 0 } Point { X = 4, Y = 6 } False

Member? maybe = null;
maybe?.Level = 9;
Console.WriteLine($"3a: {maybe is null}");

maybe = new Member();
maybe?.Level = 9;
Console.WriteLine($"3b: {maybe!.Level}");

// 実行結果
// 3a: True
// 3b: 9

Console.WriteLine($"4: {nameof(List<>)} {nameof(Dictionary<,>)} {nameof(Point)}");

// 実行結果
// 4: List Dictionary Point

private static int Sum(ReadOnlySpan<int> values)
{
    int total = 0;

    foreach (int v in values)
    {
        total += v;
    }

    return total;
}

private static string Describe<T>(ReadOnlySpan<T> values) => $"len={values.Length}";

int[] numbers = { 1, 2, 3 };
Span<int> span = numbers;
ReadOnlySpan<int> ro = span;
Console.WriteLine($"5: {Sum(numbers)} {ro.Length} {Describe(numbers)}");

// 実行結果
// 5: 6 3 len=3

private delegate bool TryParse<T>(string text, out T value);

TryParse<int> parse = (text, out value) => int.TryParse(text, out value);
Console.WriteLine($"6: {parse("42", out int n)} {n}");

// 実行結果
// 6: True 42