Getting the value of a custom attribute from an enum

Question

Suppose we have an enum called "Planet" and it has a custom attribute of class "PlanetAttr", these methods will give you the attribute value for a given Planet value:

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));
}

But is this a good way to retrieve the value of a custom attribute from an enum? I am uncomfortable with the number of method calls involved. Is there a more efficient way, e.g. can it be done without looking up the enum constant's name?

This code is from a StackOverflow question.

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 1

Make it general purpose using generics.

Though I'd strongly suggest you rename your attribute to follow .NET guidelines. It should always be in the form AttributeNameAttribute. Enum values should be in CamelCase. And it is common to name classes containing extension methods to be named the same as the class it is extending followed by Extensions.

Otherwise, that's pretty much how you'd get custom attributes of a field.

public static class EnumExtensions
{
    public static TAttribute GetAttribute<TAttribute>(this Enum value)
        where TAttribute : Attribute
    {
        var type = value.GetType();
        var name = Enum.GetName(type, value);
        return type.GetField(name) // I prefer to get attributes this way
            .GetCustomAttributes(false)
            .OfType<TAttribute>()
            .SingleOrDefault();
    }
}

public class PlanetInfoAttribute : Attribute
{
    internal PlanetInfoAttribute(double mass, double radius)
    {
        this.Mass = mass;
        this.Radius = radius;
    }
    public double Mass { get; private set; }
    public double Radius { get; private set; }
}


public enum Planet
{
    [PlanetInfo(3.303e+23, 2.43970e6)]  Mecury,
    [PlanetInfo(4.869e+24, 6.05180e6)]  Venus,
    [PlanetInfo(5.976e+24, 6.37814e6)]  Earth,
    [PlanetInfo(6.421e+23, 3.39720e6)]  Mars,
    [PlanetInfo(1.900e+27, 7.14920e7)]  Jupiter,
    [PlanetInfo(5.688e+26, 6.02680e7)]  Saturn,
    [PlanetInfo(8.686e+25, 2.55590e7)]  Uranus,
    [PlanetInfo(1.024e+26, 2.47460e7)]  Neptune,
    [PlanetInfo(1.270e+22, 1.13700e6)]  Pluto,
}

public static class PlanetExtensions
{
    public static double GetSurfaceGravity(this Planet p)
    {
        var attr = p.GetAttribute<PlanetInfoAttribute>();
        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;
}

---

It should be pointed out that there are methods to retrieve custom attributes in the framework now as of .Net 4.5. See the CustomAttributeExtensions class. This should simplify the extension method a bit. It might be worth changing the name of the extension to GetCustomAttribute() as well since it will always be a custom attribute.

public static class EnumExtensions
{
    public static TAttribute GetAttribute<TAttribute>(this Enum value)
        where TAttribute : Attribute
    {
        var type = value.GetType();
        var name = Enum.GetName(type, value);
        return type.GetField(name) // I prefer to get attributes this way
            .GetCustomAttribute<TAttribute>();
    }
}