C# vs Java Enum (for those new to C#)

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

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.

Answer 2

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

// 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 3

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 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

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 a a 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 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.