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 ...]
//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);
}
}
}
CLR中的枚举被简单命名为常量。基础类型必须是整型。在Java中，枚举更像是类型的命名实例。该类型可能相当复杂，并且—如您的示例所示—包含多个不同类型的字段。
为了将这个例子移植到c#，我只需要将枚举更改为一个不可变的类，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;
}
}
下面是另一个有趣的想法，它可以满足Java中可用的自定义行为。我提出了以下枚举基类:
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; }
}
}
它有一个类型参数，所以序数计数可以在不同的派生枚举中正常工作。Jon Skeet对上面另一个问题(http://stackoverflow.com/questions/1376312/whats-the-equivalent-of-javas-enum-in-c)的回答中的运算符示例变成:
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);
}
}
这带来了一些好处。
序数的支持
转换为字符串和int，这使得switch语句可行
GetType()将为派生的Enumeration类型的每个值提供相同的结果。
来自System的静态方法。枚举可以添加到基本枚举类以允许相同的功能。
