C#vs Java Enum(适用于C#的新手)

Question

我已经用Java编程了一段时间,只是投入了一个完全用C#编写的项目.我正试图加快C#的速度,并注意到在我的新项目中几个地方使用的枚举,但乍一看,C#的枚举似乎比Java 1.5+实现更简单.任何人都可以枚举C#和Java枚举之间的差异,以及如何克服这些差异？(我不想开始语言火焰战,我只想知道如何在C#中做一些我以前用Java做的事情).例如,有人可以发布一个C#对应的Sun着名的Planet枚举示例吗？

public enum Planet {
  MERCURY (3.303e+23, 2.4397e6),
  VENUS   (4.869e+24, 6.0518e6),
  EARTH   (5.976e+24, 6.37814e6),
  MARS    (6.421e+23, 3.3972e6),
  JUPITER (1.9e+27,   7.1492e7),
  SATURN  (5.688e+26, 6.0268e7),
  URANUS  (8.686e+25, 2.5559e7),
  NEPTUNE (1.024e+26, 2.4746e7),
  PLUTO   (1.27e+22,  1.137e6);

  private final double mass;   // in kilograms
  private final double radius; // in meters
  Planet(double mass, double radius) {
      this.mass = mass;
      this.radius = radius;
  }
  public double mass()   { return mass; }
  public double radius() { return radius; }

  // universal gravitational constant  (m3 kg-1 s-2)
  public static final double G = 6.67300E-11;

  public double surfaceGravity() {
      return G * mass / (radius * radius);
  }
  public double surfaceWeight(double otherMass) {
      return otherMass * surfaceGravity();
  }
}

// Example usage (slight modification of Sun's example):
public static void main(String[] args) {
    Planet pEarth = Planet.EARTH;
    double earthRadius = pEarth.radius(); // Just threw it in to show usage

    // Argument passed in is earth Weight.  Calculate weight on each planet:
    double earthWeight = Double.parseDouble(args[0]);
    double mass = earthWeight/pEarth.surfaceGravity();
    for (Planet p : Planet.values())
       System.out.printf("Your weight on %s is %f%n",
                         p, p.surfaceWeight(mass));
}

// Example output:
$ java Planet 175
Your weight on MERCURY is 66.107583
Your weight on VENUS is 158.374842
[etc ...]

Answer 1

在C#中,您可以在枚举上定义扩展方法,这可以弥补一些缺少的功能.

您可以定义Planet为枚举,并且还具有与surfaceGravity()和等效的扩展方法surfaceWeight().

我使用了Mikhail建议的自定义属性,但使用Dictionary可以实现相同的属性.

using System;
using System.Reflection;

class PlanetAttr: Attribute
{
    internal PlanetAttr(double mass, double radius)
    {
        this.Mass = mass;
        this.Radius = radius;
    }
    public double Mass { get; private set; }
    public double Radius { get; private set; }
}

public static class Planets
{
    public static double GetSurfaceGravity(this Planet p)
    {
        PlanetAttr attr = GetAttr(p);
        return G * attr.Mass / (attr.Radius * attr.Radius);
    }

    public static double GetSurfaceWeight(this Planet p, double otherMass)
    {
        return otherMass * p.GetSurfaceGravity();
    }

    public const double G = 6.67300E-11;

    private static PlanetAttr GetAttr(Planet p)
    {
        return (PlanetAttr)Attribute.GetCustomAttribute(ForValue(p), typeof(PlanetAttr));
    }

    private static MemberInfo ForValue(Planet p)
    {
        return typeof(Planet).GetField(Enum.GetName(typeof(Planet), p));
    }

}

public enum Planet
{
    [PlanetAttr(3.303e+23, 2.4397e6)]  MERCURY,
    [PlanetAttr(4.869e+24, 6.0518e6)]  VENUS,
    [PlanetAttr(5.976e+24, 6.37814e6)] EARTH,
    [PlanetAttr(6.421e+23, 3.3972e6)]  MARS,
    [PlanetAttr(1.9e+27,   7.1492e7)]  JUPITER,
    [PlanetAttr(5.688e+26, 6.0268e7)]  SATURN,
    [PlanetAttr(8.686e+25, 2.5559e7)]  URANUS,
    [PlanetAttr(1.024e+26, 2.4746e7)]  NEPTUNE,
    [PlanetAttr(1.27e+22,  1.137e6)]   PLUTO
}

Answer 2

CLR中的枚举只是命名常量.基础类型必须是整数.在Java中,枚举更像是类型的命名实例.该类型可能非常复杂,并且 - 如您的示例所示 - 包含多个不同类型的字段.

要将示例移植到C#,我只需将枚举更改为不可变类,并公开该类的静态只读实例:

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Planet planetEarth = Planet.MERCURY;

            double earthRadius = pEarth.Radius; // Just threw it in to show usage
            double earthWeight = double.Parse("123");
            double earthMass   = earthWeight / pEarth.SurfaceGravity();

            foreach (Planet p in Planet.Values)
                Console.WriteLine($"Your weight on {p} is {p.SurfaceWeight(mass)}");

            Console.ReadKey();
        }
    }

    public class Planet
    {
        public static readonly Planet MERCURY = new Planet("Mercury", 3.303e+23, 2.4397e6);
        public static readonly Planet VENUS   = new Planet("Venus", 4.869e+24, 6.0518e6);
        public static readonly Planet EARTH   = new Planet("Earth", 5.976e+24, 6.37814e6);
        public static readonly Planet MARS    = new Planet("Mars", 6.421e+23, 3.3972e6);
        public static readonly Planet JUPITER = new Planet("Jupiter", 1.9e+27, 7.1492e7);
        public static readonly Planet SATURN  = new Planet("Saturn", 5.688e+26, 6.0268e7);
        public static readonly Planet URANUS  = new Planet("Uranus", 8.686e+25, 2.5559e7);
        public static readonly Planet NEPTUNE = new Planet("Neptune", 1.024e+26, 2.4746e7);
        public static readonly Planet PLUTO   = new Planet("Pluto", 1.27e+22, 1.137e6);

        public static IEnumerable<Planet> Values
        {
            get
            {
                yield return MERCURY;
                yield return VENUS;
                yield return EARTH;
                yield return MARS;
                yield return JUPITER;
                yield return SATURN;
                yield return URANUS;
                yield return NEPTUNE;
                yield return PLUTO;
            }
        }

        public string Name   { get; private set; }
        public double Mass   { get; private set; }
        public double Radius { get; private set; }

        Planet(string name, double mass, double radius) => 
            (Name, Mass, Radius) = (name, mass, radius);

        // Wniversal gravitational constant  (m3 kg-1 s-2)
        public const double G = 6.67300E-11;
        public double SurfaceGravity()            => G * mass / (radius * radius);
        public double SurfaceWeight(double other) => other * SurfaceGravity();
        public override string ToString()         => name;
    }
}

Answer 3

在C#中,属性可以与枚举一起使用.这里有详细描述的编程模式的好例子(Codeproject)

public enum Planet
{
   [PlanetAttr(3.303e+23, 2.4397e6)]
   Mercury,
   [PlanetAttr(4.869e+24, 6.0518e6)]
   Venus
} 

编辑:最近又问过这个问题并由Jon Skeet回答:在C#中,Java的枚举是什么？ C#中的私有内部类 - 为什么不经常使用它们？

编辑2:看到接受的答案,以非常出色的方式扩展这种方法!

Answer 4

Java枚举实际上是完整的类,可以有私有构造函数和方法等,而C#枚举只是命名为整数.IMO Java的实现远非优越.

在学习来自java阵营的c#时,这个页面可以帮助你很多.(该链接指向枚举的差异(向上/向下滚动其他内容)

Answer 5

我认为这样的事情：

public class Planets 
{
    public static readonly Planet MERCURY = new Planet(3.303e+23, 2.4397e6);
    public static readonly Planet VENUS = new Planet(4.869e+24, 6.0518e6);
    public static readonly Planet EARTH = new Planet(5.976e+24, 6.37814e6);
    public static readonly Planet MARS = new Planet(6.421e+23, 3.3972e6);
    public static readonly Planet JUPITER = new Planet(1.9e+27,   7.1492e7);
    public static readonly Planet SATURN = new Planet(5.688e+26, 6.0268e7);
    public static readonly Planet URANUS = new Planet(8.686e+25, 2.5559e7);
    public static readonly Planet NEPTUNE = new Planet(1.024e+26, 2.4746e7);
    public static readonly Planet PLUTO = new Planet(1.27e+22,  1.137e6);
}

public class Planet
{
    public double Mass {get;private set;}
    public double Radius {get;private set;}

    Planet(double mass, double radius)
    {
        Mass = mass;
        Radius = radius;
    }

    // universal gravitational constant  (m3 kg-1 s-2)
    private static readonly double G = 6.67300E-11;

    public double SurfaceGravity()
    {
        return G * Mass / (Radius * Radius);
    }

    public double SurfaceWeight(double otherMass)
    {
        return otherMass * SurfaceGravity();
    }
}

或者将常量组合到Planet上面的类中

Answer 6

我们刚刚为 c# https://github.com/simonmau/enum_ext做了一个 enum 扩展

它只是 typesafeenum 的一个实现，但它工作得很好，所以我们制作了一个包来分享 - 玩得开心

public sealed class Weekday : TypeSafeNameEnum<Weekday, int>
{
    public static readonly Weekday Monday = new Weekday(1, "--Monday--");
    public static readonly Weekday Tuesday = new Weekday(2, "--Tuesday--");
    public static readonly Weekday Wednesday = new Weekday(3, "--Wednesday--");
    ....

    private Weekday(int id, string name) : base(id, name)
    {
    }
}