Using an enum as an array index in C#

Question

I want to do the same as in this question, that is:

enum DaysOfTheWeek {Sunday=0, Monday, Tuesday...};
string[] message_array = new string[number_of_items_at         


        

Answer 1

It was a very good answer by @ian-goldby, but it didn't address the issue raised by @zar-shardan, which is an issue I hit myself. Below is my take on a solution, with a an extension class for converting an IEnumerable, and a test class below that:

/// <summary>
/// An array indexed by an enumerated type instead of an integer
/// </summary>
public class ArrayIndexedByEnum<TKey, TElement> : IEnumerable<TElement> where TKey : Enum
{
  private readonly Array _array;
  private readonly Dictionary<TKey, TElement> _dictionary;

  /// <summary>
  /// Creates the initial array, populated with the defaults for TElement
  /// </summary>
  public ArrayIndexedByEnum()
  {
    var min = Convert.ToInt64(Enum.GetValues(typeof(TKey)).Cast<TKey>().Min());
    var max = Convert.ToInt64(Enum.GetValues(typeof(TKey)).Cast<TKey>().Max());
    var size = max - min + 1;

    // Check that we aren't creating a ridiculously big array, if we are,
    // then use a dictionary instead
    if (min >= Int32.MinValue && 
        max <= Int32.MaxValue && 
        size < Enum.GetValues(typeof(TKey)).Length * 3L)
    {
      var lowerBound = Convert.ToInt32(min);
      var upperBound = Convert.ToInt32(max);
      _array = Array.CreateInstance(typeof(TElement), new int[] {(int)size }, new int[] { lowerBound });
    }
    else
    {
      _dictionary = new Dictionary<TKey, TElement>();
      foreach (var value in Enum.GetValues(typeof(TKey)).Cast<TKey>())
      {
        _dictionary[value] = default(TElement);
      }
    }
  }

  /// <summary>
  /// Gets the element by enumerated type
  /// </summary>
  public TElement this[TKey key]
  {
    get => (TElement)(_array?.GetValue(Convert.ToInt32(key)) ?? _dictionary[key]);
    set
    {
      if (_array != null)
      {
        _array.SetValue(value, Convert.ToInt32(key));
      }
      else
      {
        _dictionary[key] = value;
      }
    }
  }

  /// <summary>
  /// Gets a generic enumerator
  /// </summary>
  public IEnumerator<TElement> GetEnumerator()
  {
    return Enum.GetValues(typeof(TKey)).Cast<TKey>().Select(k => this[k]).GetEnumerator();
  }

  System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
  {
    return GetEnumerator();
  }
}

Here's the extension class:

/// <summary>
/// Extensions for converting IEnumerable<TElement> to ArrayIndexedByEnum
/// </summary>
public static class ArrayIndexedByEnumExtensions
{
  /// <summary>
  /// Creates a ArrayIndexedByEnumExtensions from an System.Collections.Generic.IEnumerable
  /// according to specified key selector and element selector functions.
  /// </summary>
  public static ArrayIndexedByEnum<TKey, TElement> ToArrayIndexedByEnum<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector) where TKey : Enum
  {
    var array = new ArrayIndexedByEnum<TKey, TElement>();
    foreach(var item in source)
    {
      array[keySelector(item)] = elementSelector(item);
    }
    return array;
  }
  /// <summary>
  /// Creates a ArrayIndexedByEnum from an System.Collections.Generic.IEnumerable
  /// according to a specified key selector function.
  /// </summary>
  public static ArrayIndexedByEnum<TKey, TSource> ToArrayIndexedByEnum<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) where TKey : Enum
  {
    return source.ToArrayIndexedByEnum(keySelector, i => i);
  }
}

And here are my tests:

[TestClass]
public class ArrayIndexedByEnumUnitTest
{
  private enum OddNumbersEnum : UInt16
  {
    One = 1,
    Three = 3,
    Five = 5,
    Seven = 7,
    Nine = 9
  }

  private enum PowersOf2 : Int64
  {
    TwoP0 = 1,
    TwoP1 = 2,
    TwoP2 = 4,
    TwoP3 = 8,
    TwoP4 = 16,
    TwoP5 = 32,
    TwoP6 = 64,
    TwoP7 = 128,
    TwoP8 = 256,
    TwoP9 = 512,
    TwoP10 = 1_024,
    TwoP11 = 2_048,
    TwoP12 = 4_096,
    TwoP13 = 8_192,
    TwoP14 = 16_384,
    TwoP15 = 32_768,
    TwoP16 = 65_536,
    TwoP17 = 131_072,
    TwoP18 = 262_144,
    TwoP19 = 524_288,
    TwoP20 = 1_048_576,
    TwoP21 = 2_097_152,
    TwoP22 = 4_194_304,
    TwoP23 = 8_388_608,
    TwoP24 = 16_777_216,
    TwoP25 = 33_554_432,
    TwoP26 = 67_108_864,
    TwoP27 = 134_217_728,
    TwoP28 = 268_435_456,
    TwoP29 = 536_870_912,
    TwoP30 = 1_073_741_824,
    TwoP31 = 2_147_483_648,
    TwoP32 = 4_294_967_296,
    TwoP33 = 8_589_934_592,
    TwoP34 = 17_179_869_184,
    TwoP35 = 34_359_738_368,
    TwoP36 = 68_719_476_736,
    TwoP37 = 137_438_953_472,
    TwoP38 = 274_877_906_944,
    TwoP39 = 549_755_813_888,
    TwoP40 = 1_099_511_627_776,
    TwoP41 = 2_199_023_255_552,
    TwoP42 = 4_398_046_511_104,
    TwoP43 = 8_796_093_022_208,
    TwoP44 = 17_592_186_044_416,
    TwoP45 = 35_184_372_088_832,
    TwoP46 = 70_368_744_177_664,
    TwoP47 = 140_737_488_355_328,
    TwoP48 = 281_474_976_710_656,
    TwoP49 = 562_949_953_421_312,
    TwoP50 = 1_125_899_906_842_620,
    TwoP51 = 2_251_799_813_685_250,
    TwoP52 = 4_503_599_627_370_500,
    TwoP53 = 9_007_199_254_740_990,
    TwoP54 = 18_014_398_509_482_000,
    TwoP55 = 36_028_797_018_964_000,
    TwoP56 = 72_057_594_037_927_900,
    TwoP57 = 144_115_188_075_856_000,
    TwoP58 = 288_230_376_151_712_000,
    TwoP59 = 576_460_752_303_423_000,
    TwoP60 = 1_152_921_504_606_850_000,
  }

  [TestMethod]
  public void TestSimpleArray()
  {
    var array = new ArrayIndexedByEnum<OddNumbersEnum, string>();

    var odds = Enum.GetValues(typeof(OddNumbersEnum)).Cast<OddNumbersEnum>().ToList();

    // Store all the values
    foreach (var odd in odds)
    {
      array[odd] = odd.ToString();
    }

    // Check the retrieved values are the same as what was stored
    foreach (var odd in odds)
    {
      Assert.AreEqual(odd.ToString(), array[odd]);
    }
  }

  [TestMethod]
  public void TestPossiblyHugeArray()
  {
    var array = new ArrayIndexedByEnum<PowersOf2, string>();

    var powersOf2s = Enum.GetValues(typeof(PowersOf2)).Cast<PowersOf2>().ToList();

    // Store all the values
    foreach (var powerOf2 in powersOf2s)
    {
      array[powerOf2] = powerOf2.ToString();
    }

    // Check the retrieved values are the same as what was stored
    foreach (var powerOf2 in powersOf2s)
    {
      Assert.AreEqual(powerOf2.ToString(), array[powerOf2]);
    }
  }
}

Answer 2

Since C# 7.3 it has been possible to use System.Enum as a constraint on type parameters. So the nasty hacks in the some of the other answers are no longer required.

Here's a very simple ArrayByEum class that does exactly what the question asked.

Note that it will waste space if the enum values are non-contiguous, and won't cope with enum values that are too large for an int. I did say this example was very simple.

/// <summary>An array indexed by an Enum</summary>
/// <typeparam name="T">Type stored in array</typeparam>
/// <typeparam name="U">Indexer Enum type</typeparam>
public class ArrayByEnum<T,U> : IEnumerable where U : Enum // requires C# 7.3 or later
{
  private readonly T[] _array;
  private readonly int _lower;

  public ArrayByEnum()
  {
    _lower = Convert.ToInt32(Enum.GetValues(typeof(U)).Cast<U>().Min());
    int upper = Convert.ToInt32(Enum.GetValues(typeof(U)).Cast<U>().Max());
    _array = new T[1 + upper - _lower];
  }

  public T this[U key]
  {
    get { return _array[Convert.ToInt32(key) - _lower]; }
    set { _array[Convert.ToInt32(key) - _lower] = value; }
  }

  public IEnumerator GetEnumerator()
  {
    return Enum.GetValues(typeof(U)).Cast<U>().Select(i => this[i]).GetEnumerator();
  }
}

Usage:

ArrayByEnum<string,MyEnum> myArray = new ArrayByEnum<string,MyEnum>();
myArray[MyEnum.First] = "Hello";

myArray[YourEnum.Other] = "World"; // compiler error

Answer 3

Here you go:

string[] message_array = Enum.GetNames(typeof(DaysOfTheWeek));

If you really need the length, then just take the .Length on the result :) You can get values with:

string[] message_array = Enum.GetValues(typeof(DaysOfTheWeek));

Answer 4

I realize this is an old question, but there have been a number of comments about the fact that all solutions so far have run-time checks to ensure the data type is an enum. Here is a complete solution (with some examples) of a solution with compile time checks (as well as some comments and discussions from my fellow developers)

//There is no good way to constrain a generic class parameter to an Enum.  The hack below does work at compile time,
//  though it is convoluted.  For examples of how to use the two classes EnumIndexedArray and ObjEnumIndexedArray,
//  see AssetClassArray below.  Or, e.g.
//      EConstraint.EnumIndexedArray<int, YourEnum> x = new EConstraint.EnumIndexedArray<int, YourEnum>();
//  See this post 
//      http://stackoverflow.com/questions/79126/create-generic-method-constraining-t-to-an-enum/29581813#29581813
// and the answer/comments by Julien Lebosquain
public class EConstraint : HackForCompileTimeConstraintOfTEnumToAnEnum<System.Enum> { }//THIS MUST BE THE ONLY IMPLEMENTATION OF THE ABSTRACT HackForCompileTimeConstraintOfTEnumToAnEnum
public abstract class HackForCompileTimeConstraintOfTEnumToAnEnum<SystemEnum> where SystemEnum : class
{
    //For object types T, users should use EnumIndexedObjectArray below.
    public class EnumIndexedArray<T, TEnum>
        where TEnum : struct, SystemEnum
    {
        //Needs to be public so that we can easily do things like intIndexedArray.data.sum()
        //   - just not worth writing up all the equivalent methods, and we can't inherit from T[] and guarantee proper initialization.
        //Also, note that we cannot use Length here for initialization, even if Length were defined the same as GetNumEnums up to
        //  static qualification, because we cannot use a non-static for initialization here.
        //  Since we want Length to be non-static, in keeping with other definitions of the Length property, we define the separate static
        //  GetNumEnums, and then define the non-static Length in terms of the actual size of the data array, just for clarity,
        //  safety and certainty (in case someone does something stupid like resizing data).
        public T[] data = new T[GetNumEnums()];

        //First, a couple of statics allowing easy use of the enums themselves.
        public static TEnum[] GetEnums()
        {
            return (TEnum[])Enum.GetValues(typeof(TEnum));
        }
        public TEnum[] getEnums()
        {
            return GetEnums();
        }
        //Provide a static method of getting the number of enums.  The Length property also returns this, but it is not static and cannot be use in many circumstances.
        public static int GetNumEnums()
        {
            return GetEnums().Length;
        }
        //This should always return the same as GetNumEnums, but is not static and does it in a way that guarantees consistency with the member array.
        public int Length { get { return data.Length; } }
        //public int Count  { get { return data.Length; } }

        public EnumIndexedArray() { }

        // [WDS 2015-04-17] Remove. This can be dangerous. Just force people to use EnumIndexedArray(T[] inputArray).
        // [DIM 2015-04-18] Actually, if you think about it, EnumIndexedArray(T[] inputArray) is just as dangerous:
        //   For value types, both are fine.  For object types, the latter causes each object in the input array to be referenced twice,
        //   while the former causes the single object t to be multiply referenced.  Two references to each of many is no less dangerous
        //   than 3 or more references to one. So all of these are dangerous for object types.
        //   We could remove all these ctors from this base class, and create a separate
        //         EnumIndexedValueArray<T, TEnum> : EnumIndexedArray<T, TEnum> where T: struct ...
        //   but then specializing to TEnum = AssetClass would have to be done twice below, once for value types and once
        //   for object types, with a repetition of all the property definitions.  Violating the DRY principle that much
        //   just to protect against stupid usage, clearly documented as dangerous, is not worth it IMHO.
        public EnumIndexedArray(T t)
        {
            int i = Length;
            while (--i >= 0)
            {
                this[i] = t;
            }
        }
        public EnumIndexedArray(T[] inputArray)
        {
            if (inputArray.Length > Length)
            {
                throw new Exception(string.Format("Length of enum-indexed array ({0}) to big. Can't be more than {1}.", inputArray.Length, Length));
            }
            Array.Copy(inputArray, data, inputArray.Length);
        }
        public EnumIndexedArray(EnumIndexedArray<T, TEnum> inputArray)
        {
            Array.Copy(inputArray.data, data, data.Length);
        }

        //Clean data access
        public T this[int ac] { get { return data[ac]; } set { data[ac] = value; } }
        public T this[TEnum ac] { get { return data[Convert.ToInt32(ac)]; } set { data[Convert.ToInt32(ac)] = value; } }
    }


    public class EnumIndexedObjectArray<T, TEnum> : EnumIndexedArray<T, TEnum>
        where TEnum : struct, SystemEnum
        where T : new()
    {
        public EnumIndexedObjectArray(bool doInitializeWithNewObjects = true)
        {
            if (doInitializeWithNewObjects)
            {
                for (int i = Length; i > 0; this[--i] = new T()) ;
            }
        }
        // The other ctor's are dangerous for object arrays
    }

    public class EnumIndexedArrayComparator<T, TEnum> : EqualityComparer<EnumIndexedArray<T, TEnum>>
        where TEnum : struct, SystemEnum
    {
        private readonly EqualityComparer<T> elementComparer = EqualityComparer<T>.Default;

        public override bool Equals(EnumIndexedArray<T, TEnum> lhs, EnumIndexedArray<T, TEnum> rhs)
        {
            if (lhs == rhs)
                return true;
            if (lhs == null || rhs == null)
                return false;

            //These cases should not be possible because of the way these classes are constructed.
            // HOWEVER, the data member is public, so somebody _could_ do something stupid and make 
            // data=null, or make lhs.data == rhs.data, even though lhs!=rhs (above check)
            //On the other hand, these are just optimizations, so it won't be an issue if we reomve them anyway,
            // Unless someone does something really dumb like setting .data to null or resizing to an incorrect size,
            // in which case things will crash, but any developer who does this deserves to have it crash painfully...
            //if (lhs.data == rhs.data)
            //    return true;
            //if (lhs.data == null || rhs.data == null)
            //    return false;

            int i = lhs.Length;
            //if (rhs.Length != i)
            //    return false;
            while (--i >= 0)
            {
                if (!elementComparer.Equals(lhs[i], rhs[i]))
                    return false;
            }
            return true;
        }
        public override int GetHashCode(EnumIndexedArray<T, TEnum> enumIndexedArray)
        {
            //This doesn't work: for two arrays ar1 and ar2, ar1.GetHashCode() != ar2.GetHashCode() even when ar1[i]==ar2[i] for all i (unless of course they are the exact same array object)
            //return engineArray.GetHashCode();
            //Code taken from comment by Jon Skeet - of course - in http://stackoverflow.com/questions/7244699/gethashcode-on-byte-array
            //31 and 17 are used commonly elsewhere, but maybe because everyone is using Skeet's post.
            //On the other hand, this is really not very critical.
            unchecked
            {
                int hash = 17;
                int i = enumIndexedArray.Length;
                while (--i >= 0)
                {
                    hash = hash * 31 + elementComparer.GetHashCode(enumIndexedArray[i]);
                }
                return hash;
            }
        }
    }
}

//Because of the above hack, this fails at compile time - as it should.  It would, otherwise, only fail at run time.
//public class ThisShouldNotCompile : EConstraint.EnumIndexedArray<int, bool>
//{
//}

//An example
public enum AssetClass { Ir, FxFwd, Cm, Eq, FxOpt, Cr };
public class AssetClassArrayComparator<T> : EConstraint.EnumIndexedArrayComparator<T, AssetClass> { }
public class AssetClassIndexedArray<T> : EConstraint.EnumIndexedArray<T, AssetClass>
{
    public AssetClassIndexedArray()
    {
    }
    public AssetClassIndexedArray(T t) : base(t)
    {
    }
    public AssetClassIndexedArray(T[] inputArray) :  base(inputArray)
    {
    }
    public AssetClassIndexedArray(EConstraint.EnumIndexedArray<T, AssetClass> inputArray) : base(inputArray)
    {
    }

    public T Cm    { get { return this[AssetClass.Cm   ]; } set { this[AssetClass.Cm   ] = value; } }
    public T FxFwd { get { return this[AssetClass.FxFwd]; } set { this[AssetClass.FxFwd] = value; } }
    public T Ir    { get { return this[AssetClass.Ir   ]; } set { this[AssetClass.Ir   ] = value; } }
    public T Eq    { get { return this[AssetClass.Eq   ]; } set { this[AssetClass.Eq   ] = value; } }
    public T FxOpt { get { return this[AssetClass.FxOpt]; } set { this[AssetClass.FxOpt] = value; } }
    public T Cr    { get { return this[AssetClass.Cr   ]; } set { this[AssetClass.Cr   ] = value; } }
}

//Inherit from AssetClassArray<T>, not EnumIndexedObjectArray<T, AssetClass>, so we get the benefit of the public access getters and setters above
public class AssetClassIndexedObjectArray<T> : AssetClassIndexedArray<T> where T : new()
{
    public AssetClassIndexedObjectArray(bool bInitializeWithNewObjects = true)
    {
        if (bInitializeWithNewObjects)
        {
            for (int i = Length; i > 0; this[--i] = new T()) ;
        }
    }
}

EDIT: If you are using C# 7.3 or later, PLEASE don't use this ugly solution. See Ian Goldby's answer from 2018.