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

If all you need is essentially a map, but don't want to incur performance overhead associated with dictionary lookups, this might work:

    public class EnumIndexedArray : IEnumerable> where TKey : struct
    {
        public EnumIndexedArray()
        {
            if (!typeof (TKey).IsEnum) throw new InvalidOperationException("Generic type argument is not an Enum");
            var size = Convert.ToInt32(Keys.Max()) + 1;
            Values = new T[size];
        }

        protected T[] Values;

        public static IEnumerable Keys
        {
            get { return Enum.GetValues(typeof (TKey)).OfType(); }
        }

        public T this[TKey index]
        {
            get { return Values[Convert.ToInt32(index)]; }
            set { Values[Convert.ToInt32(index)] = value; }
        }

        private IEnumerable> CreateEnumerable()
        {
            return Keys.Select(key => new KeyValuePair(key, Values[Convert.ToInt32(key)]));
        }

        public IEnumerator> GetEnumerator()
        {
            return CreateEnumerable().GetEnumerator();
        }

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

So in your case you could derive:

class DaysOfWeekToStringsMap:EnumIndexedArray{};

Usage:

var map = new DaysOfWeekToStringsMap();

//using the Keys static property
foreach(var day in DaysOfWeekToStringsMap.Keys){
    map[day] = day.ToString();
}
foreach(var day in DaysOfWeekToStringsMap.Keys){
    Console.WriteLine("map[{0}]={1}",day, map[day]);
}

// using iterator
foreach(var value in map){
    Console.WriteLine("map[{0}]={1}",value.Key, value.Value);
}

Obviously this implementation is backed by an array, so non-contiguous enums like this:

enum
{
  Ok = 1,
  NotOk = 1000000
}

would result in excessive memory usage.

If you require maximum possible performance you might want to make it less generic and loose all generic enum handling code I had to use to get it to compile and work. I didn't benchmark this though, so maybe it's no big deal.

Caching the Keys static property might also help.