C＃vs Java Enum（适用于C＃的新手）

Question

I've been programming in Java for a while and just got thrown onto a project that's written entirely in C#. I'm trying to come up to speed in C#, and noticed enums used in several places in my new project, but at first glance, C#'s enums seem to be more simplistic than the Java 1.5+ implementation. Can anyone enumerate the differences between C# and Java enums, and how to overcome the differences? (I don't want to start a language flame war, I just want to know how to do some things in C# that I used to do in Java). For example, could someone post a C# counterpart to Sun's famous Planet enum example?

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

In C# you can define extension methods on enums, and this makes up for some of the missing functionality.

You can define Planet as an enum and also have extension methods equivalent to surfaceGravity() and surfaceWeight() .

I have used custom attributes as suggested by Mikhail , but the same could be achieved using a Dictionary.

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

Enumerations in the CLR are simply named constants. The underlying type must be integral. In Java an enumeration is more like a named instance of a type. That type can be quite complex and - as your example shows - contain multiple fields of various types.

To port the example to C# I would just change the enum to an immutable class and expose static readonly instances of that class:

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

In C# attributes can be used with enums. Good example of this programming pattern with detailed description is here (Codeproject)

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

Edit: this question has been recently asked again and answered by Jon Skeet: What's the equivalent of Java's enum in C#? Private inner classes in C# - why aren't they used more often?

Edit 2: see the accepted answer which extends this approach in a very brilliant way!

Answer 4

Java enums are actually full classes which can have a private constructor and methods etc, whereas C# enums are just named integers. IMO Java's implementation is far superior.

This page should help you a lot while learning c# coming from a java camp. (The link points to the differences about enums (scroll up / down for other things)

Answer 5

Something like this I think:

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();
    }
}

Or combine the constants into the Planet class as above

Answer 6

Here's another interesting idea which caters for the custom behaviour available in Java. I came up with the following Enumeration base class:

public abstract class Enumeration<T>
    where T : Enumeration<T>
{   
    protected static int nextOrdinal = 0;

    protected static readonly Dictionary<int, Enumeration<T>> byOrdinal = new Dictionary<int, Enumeration<T>>();
    protected static readonly Dictionary<string, Enumeration<T>> byName = new Dictionary<string, Enumeration<T>>();

    protected readonly string name;
    protected readonly int ordinal;

    protected Enumeration(string name)
        : this (name, nextOrdinal)
    {
    }

    protected Enumeration(string name, int ordinal)
    {
        this.name = name;
        this.ordinal = ordinal;
        nextOrdinal = ordinal + 1;
        byOrdinal.Add(ordinal, this);
        byName.Add(name, this);
    }

    public override string ToString()
    {
        return name;
    }

    public string Name 
    {
        get { return name; }
    }

    public static explicit operator int(Enumeration<T> obj)
    {
        return obj.ordinal;
    }

    public int Ordinal
    {
        get { return ordinal; }
    }
}

It's got a type parameter basically just so the ordinal count will work properly across different derived enumerations. Jon Skeet's Operator example from his answer to another question (http://stackoverflow.com/questions/1376312/whats-the-equivalent-of-javas-enum-in-c) above then becomes:

public class Operator : Enumeration<Operator>
{
    public static readonly Operator Plus = new Operator("Plus", (x, y) => x + y);
    public static readonly Operator Minus =  new Operator("Minus", (x, y) => x - y);
    public static readonly Operator Times =  new Operator("Times", (x, y) => x * y);
    public static readonly Operator Divide = new Operator("Divide", (x, y) => x / y);

    private readonly Func<int, int, int> op;

    // Prevent other top-level types from instantiating
    private Operator(string name, Func<int, int, int> op)
        :base (name)
    {
        this.op = op;
    }

    public int Execute(int left, int right)
    {
        return op(left, right);
    }
}

This gives a few advantages.

• Ordinal support
• Conversion to string and int which makes switch statements feasible
• GetType() will give the same result for each of the values of a derived Enumeration type.
• The Static methods from System.Enum can be added to the base Enumeration class to allow the same functionality.

Answer 7

A Java enum is syntactic sugar to present enumerations in an OO manner. They're abstract classes extending the Enum class in Java, and each enum value is like a static final public instance implementation of the enum class. Look at the generated classes, and for an enum "Foo" with 10 values, you'll see "Foo$1" through "Foo$10" classes generated.

I don't know C# though, I can only speculate that an enum in that language is more like a traditional enum in C style languages. I see from a quick Google search that they can hold multiple values however, so they are probably implemented in a similar manner, but with far more restrictions than what the Java compiler allows.

Answer 8

Java enums allow easy typesafe conversions from the name using the compiler-generated valueOf method, ie

// Java Enum has generics smarts and allows this
Planet p = Planet.valueOf("MERCURY");

The equivalent for a raw enum in C# is more verbose:

// C# enum - bit of hoop jumping required
Planet p = (Planet)Enum.Parse(typeof(Planet), "MERCURY");

However, if you go down the route sugegsted by Kent, you can easily implement a ValueOf method in your enum class.

Answer 9

we have just made an enum extension for c# https://github.com/simonmau/enum_ext

It's just a implementation for the typesafeenum, but it works great so we made a package to share - have fun with it

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)
    {
    }
}

Answer 10

I suspect enums in C# are just constants internal to the CLR, but not that familiar with them. I have decompiled some classes in Java and I can tell you want Enums are once you convert.

Java does something sneaky. It treats the enum class as aa normal class with, as close as I can figure, using lots of macros when referencing the enum values. If you have a case statement in a Java class that uses enums, it replaces the enum references to integers. If you need to go to string, it creates an array of strings indexed by an ordinal that it uses in each class. I suspect to save on boxing.

If you download this decompiler you will get to see how it creates its class an integrates it. Rather fascinating to be honest. I used to not use the enum class because I thought it was to bloated for just an array of constants. I like it better than the limited way you can use them in C#.

http://members.fortunecity.com/neshkov/dj.html -- Java decompiler

Answer 11

//Review the sample enum below for a template on how to implement a JavaEnum.
//There is also an EnumSet implementation below.

public abstract class JavaEnum : IComparable {
    public static IEnumerable<JavaEnum> Values {
        get {
            throw new NotImplementedException("Enumeration missing");
        }
    }

    public readonly string Name;

    public JavaEnum(string name) {
        this.Name = name;
    }

    public override string ToString() {
        return base.ToString() + "." + Name.ToUpper();
    }

    public int CompareTo(object obj) {
        if(obj is JavaEnum) {
            return string.Compare(this.Name, ((JavaEnum)obj).Name);
        } else {
            throw new ArgumentException();
        }
    }


    //Dictionary values are of type SortedSet<T>
    private static Dictionary<Type, object> enumDictionary;
    public static SortedSet<T> RetrieveEnumValues<T>() where T : JavaEnum {
        if(enumDictionary == null) {
            enumDictionary = new Dictionary<Type, object>();
        }
        object enums;
        if(!enumDictionary.TryGetValue(typeof(T), out enums)) {
            enums = new SortedSet<T>();
            FieldInfo[] myFieldInfo = typeof(T).GetFields(BindingFlags.Static | BindingFlags.DeclaredOnly | BindingFlags.Public);
            foreach(FieldInfo f in myFieldInfo) {
                if(f.FieldType == typeof(T)) {
                    ((SortedSet<T>)enums).Add((T)f.GetValue(null));
                }
            }
            enumDictionary.Add(typeof(T), enums);
        }
        return (SortedSet<T>)enums;
    }
}


//Sample JavaEnum
public class SampleEnum : JavaEnum {
    //Enum values
    public static readonly SampleEnum A = new SampleEnum("A", 1);
    public static readonly SampleEnum B = new SampleEnum("B", 2);
    public static readonly SampleEnum C = new SampleEnum("C", 3);

    //Variables or Properties common to all enums of this type
    public int int1;
    public static int int2 = 4;
    public static readonly int int3 = 9;

    //The Values property must be replaced with a call to JavaEnum.generateEnumValues<MyEnumType>() to generate an IEnumerable set.
    public static new IEnumerable<SampleEnum> Values {
        get {
            foreach(var e in JavaEnum.RetrieveEnumValues<SampleEnum>()) {
                yield return e;
            }
            //If this enum should compose several enums, add them here
            //foreach(var e in ChildSampleEnum.Values) {
            //    yield return e;
            //}
        }
    }

    public SampleEnum(string name, int int1)
        : base(name) {
        this.int1 = int1;
    }
}


public class EnumSet<T> : SortedSet<T> where T : JavaEnum {
    // Creates an enum set containing all of the elements in the specified element type.
    public static EnumSet<T> AllOf(IEnumerable<T> values) {
        EnumSet<T> returnSet = new EnumSet<T>();
        foreach(T item in values) {
            returnSet.Add(item);
        }
        return returnSet;
    }

    // Creates an enum set with the same element type as the specified enum set, initially containing all the elements of this type that are not contained in the specified set.
    public static EnumSet<T> ComplementOf(IEnumerable<T> values, EnumSet<T> set) {
        EnumSet<T> returnSet = new EnumSet<T>();
        foreach(T item in values) {
            if(!set.Contains(item)) {
                returnSet.Add(item);
            }
        }
        return returnSet;
    }

    // Creates an enum set initially containing all of the elements in the range defined by the two specified endpoints.
    public static EnumSet<T> Range(IEnumerable<T> values, T from, T to) {
        EnumSet<T> returnSet = new EnumSet<T>();
        if(from == to) {
            returnSet.Add(from);
            return returnSet;
        }
        bool isFrom = false;
        foreach(T item in values) {
            if(isFrom) {
                returnSet.Add(item);
                if(item == to) {
                    return returnSet;
                }
            } else if(item == from) {
                isFrom = true;
                returnSet.Add(item);
            }
        }
        throw new ArgumentException();
    }

    // Creates an enum set initially containing the specified element(s).
    public static EnumSet<T> Of(params T[] setItems) {
        EnumSet<T> returnSet = new EnumSet<T>();
        foreach(T item in setItems) {
            returnSet.Add(item);
        }
        return returnSet;
    }

    // Creates an empty enum set with the specified element type.
    public static EnumSet<T> NoneOf() {
        return new EnumSet<T>();
    }

    // Returns a copy of the set passed in.
    public static EnumSet<T> CopyOf(EnumSet<T> set) {
        EnumSet<T> returnSet = new EnumSet<T>();
        returnSet.Add(set);
        return returnSet;
    }

    // Adds a set to an existing set.
    public void Add(EnumSet<T> enumSet) {
        foreach(T item in enumSet) {
            this.Add(item);
        }
    }

    // Removes a set from an existing set.
    public void Remove(EnumSet<T> enumSet) {
        foreach(T item in enumSet) {
            this.Remove(item);
        }
    }
}

Answer 12

You could also use a utility class for each enum type which holds a instance with advanced data for each enum value.

public enum Planet
{
    MERCURY,
    VENUS
}

public class PlanetUtil
{
    private static readonly IDictionary<Planet, PlanetUtil> PLANETS = new Dictionary<Planet, PlanetUtil();

    static PlanetUtil()
    {
        PlanetUtil.PLANETS.Add(Planet.MERCURY, new PlanetUtil(3.303e+23, 2.4397e6));
        PlanetUtil.PLANETS.Add(Planet.VENUS, new PlanetUtil(4.869e+24, 6.0518e6));
    }

    public static PlanetUtil GetUtil(Planet planet)
    {
        return PlanetUtil.PLANETS[planet];
    }

    private readonly double radius;
    private readonly double mass;

    public PlanetUtil(double radius, double mass)
    {
        this.radius = radius;
        this.mass = mass;
    }

    // getter
}

Answer 13

The enum in Java is much more complex than C# enum and hence more powerful. Since it is just another compile time syntactical sugar I'm wondering if it was really worth having included the language given its limited usage in real life applications. Sometimes it's harder keeping stuff out of the language than giving up to the pressure to include a minor feature.