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

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 } 

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!

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)

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

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) { } } 

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.

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.

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.

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

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.

//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); } } } 

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 }