switch式とwhenによる網羅性チェック（C# 6でのエミュレーション）

計画: switch のエミュレーション

値を返す switch

using System;

public enum Grade { A, B, C, D, F }

public class Program
{
  static string Describe(Grade g)
  {
    switch (g)
    {
      case Grade.A: return "Excellent";
      case Grade.B: return "Good";
      case Grade.C: return "Average";
      case Grade.D: return "Below average";
      case Grade.F: return "Fail";
      default: throw new ArgumentOutOfRangeException("g");
    }
  }

  public static void Main(string[] args)
  {
    Console.WriteLine(Describe(Grade.A));
    Console.WriteLine(Describe(Grade.F));
  }
}

「when」をガードで表現

using System;

public class Program
{
  // Emulate: switch(score) { case var s when s >= 90: ... } using pre-guards
  static string LabelScore(int score)
  {
    if (score < 0 || score > 100) throw new ArgumentOutOfRangeException("score");

    if (score >= 90) return "A";
    if (score >= 80) return "B";
    if (score >= 70) return "C";
    if (score >= 60) return "D";
    return "F";
  }

  public static void Main(string[] args)
  {
    Console.WriteLine(LabelScore(92)); // A
    Console.WriteLine(LabelScore(77)); // C
  }
}

網羅性ヘルパー

using System;

public enum Status { New, Active, Closed }

public static class Exhaustive
{
  public static Exception Unhandled(object x)
  {
    return new InvalidOperationException("Unhandled case: " + x);
  }
}

public class Program
{
  static int ToHttp(Status s)
  {
    switch (s)
    {
      case Status.New: return 201;
      case Status.Active: return 200;
      case Status.Closed: return 410;
      default: throw Exhaustive.Unhandled(s);
    }
  }

  public static void Main(string[] args)
  {
    Console.WriteLine(ToHttp(Status.Active)); // 200
  }
}

ガード＋kindによるswitch

using System;

public enum ShapeKind { Circle, Square }

public sealed class Shape
{
  public ShapeKind Kind;
  public double Radius; // for circle
  public double Side;   // for square
}

public class Program
{
  static double Area(Shape s)
  {
    if (s == null) throw new ArgumentNullException("s");

    // “when” guards (validate needed fields)
    if (s.Kind == ShapeKind.Circle && s.Radius < 0) throw new ArgumentException("Radius must be >= 0");
    if (s.Kind == ShapeKind.Square && s.Side < 0) throw new ArgumentException("Side must be >= 0");

    switch (s.Kind)
    {
      case ShapeKind.Circle: return Math.PI * s.Radius * s.Radius;
      case ShapeKind.Square: return s.Side * s.Side;
      default: throw new InvalidOperationException("Unhandled shape: " + s.Kind);
    }
  }

  public static void Main(string[] args)
  {
    Shape c = new Shape { Kind = ShapeKind.Circle, Radius = 2.0 };
    Console.WriteLine("Area circle = " + Area(c));
  }
}

ヒントと注意点

網羅性の再現

振り返り