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:

/// 

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

  /// 

  /// Creates the initial array, populated with the defaults for TElement
  /// 
  public ArrayIndexedByEnum()
  {
    var min = Convert.ToInt64(Enum.GetValues(typeof(TKey)).Cast().Min());
    var max = Convert.ToInt64(Enum.GetValues(typeof(TKey)).Cast().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();
      foreach (var value in Enum.GetValues(typeof(TKey)).Cast())
      {
        _dictionary[value] = default(TElement);
      }
    }
  }

  /// 

  /// Gets the element by enumerated type
  /// 
  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;
      }
    }
  }

  /// 

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

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

Here's the extension class:

/// 

/// Extensions for converting IEnumerable to ArrayIndexedByEnum
/// 
public static class ArrayIndexedByEnumExtensions
{
  /// 

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

  /// Creates a ArrayIndexedByEnum from an System.Collections.Generic.IEnumerable
  /// according to a specified key selector function.
  /// 
  public static ArrayIndexedByEnum ToArrayIndexedByEnum(this IEnumerable source, Func 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();

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

    var powersOf2s = Enum.GetValues(typeof(PowersOf2)).Cast().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]);
    }
  }
}