C# vs. Java Enums: How Do I Add Functionality to C# Enums Like in Java?

C# and Java Enumerations: Difference Analysis

For developers moving from Java to C#, it is critical to understand the differences between enumerations in the two languages. While C# enums look cleaner, they initially lack some of Java's advanced features.

Main Differences

• Constructor parameters: Java enumerations can have constructor parameters, allowing initialization with additional data. C# enums have no constructors.
• Methods and Properties: Java enumerations can contain methods and properties. C# enumerations cannot define methods or properties directly; they must be defined as extension methods.
• Annotation equivalent: In Java, you can use annotations to associate metadata with an enumeration. In C#, custom attributes have similar functionality.

Bridging the differences

• Extension methods: C# extension methods provide the functionality of Java enumeration methods and properties.
• Custom Attributes: C# custom attributes can be used to store additional data next to the enumeration value.

Java Planet enumeration example in C#

The following C# code demonstrates an equivalent implementation of Sun's Planet enumeration, including extension methods and custom attributes:

using System;
using System.Reflection;

namespace PlanetEnum
{
    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));
        }
    }

    [AttributeUsage(AttributeTargets.Field)]
    public class PlanetAttr : Attribute
    {
        public PlanetAttr(double mass, double radius)
        {
            Mass = mass;
            Radius = radius;
        }

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

    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
    }
}

This code demonstrates how to use extension methods and custom attributes to extend C# enums with additional functionality, effectively bridging the gap with Java enums.

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