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

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.

/// 
An array indexed by an Enum
/// Type stored in array
/// Indexer Enum type
public class ArrayByEnum : 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().Min());
    int upper = Convert.ToInt32(Enum.GetValues(typeof(U)).Cast().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().Select(i => this[i]).GetEnumerator();
  }
}

Usage:

ArrayByEnum myArray = new ArrayByEnum();
myArray[MyEnum.First] = "Hello";

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