Indexers

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace csharp
{
    public class Foo
    {
        private string[] s;
        private int len;

        public Foo(int l)
        {
            len = l;
            s = new string[l];
        }

        public string this[int i]
        {
            get
            {
                return s[i];
            }

            set
            {
                s[i] = value;
            }
        }

        public string this[string str, int m = 1]
        {
            get
            {
                StringBuilder sb = new StringBuilder();

                if (m > 1)
                {
                    for (int i = 0; i < len; i++)
                    {
                        if (s[i] == str)
                            sb.Append(i.ToString() + ",");
                    }
                    sb.Remove(sb.Length - 1, 1);
                }
                else
                {
                    for (int i = 0; i < len; i++)
                    {
                        if (s[i] == str)
                        {
                            sb.Append(i.ToString());
                            break;
                        }
                    }
                }

                return sb.ToString();
            }

            set
            {
                if (m > 1)
                {
                    for (int i = 0; i < len; i++)
                    {
                        if (s[i] == str)
                            s[i] = value;
                    }
                }
                else
                {
                    for (int i = 0; i < len; i++)
                    {
                        if (s[i] == str)
                        {
                            s[i] = value;
                            return;
                        }
                    }
                }
            }
        }
    }

    public class Program
    {
        static void Main(string[] args)
        {
            Foo f = new Foo(5);

            f[0] = "null";
            f[1] = "eins";
            f[2] = "zwei";
            f[3] = "drei";
            f[4] = "drei";

            for (int i = 0; i < 5; i++)
                Console.WriteLine(f[i]);

            f["drei", 2] = "DREI";

            for (int i = 0; i < 5; i++)
                Console.WriteLine(f[i]);

            Console.WriteLine(f["DREI"]);
            Console.WriteLine(f["DREI", 2]);

            f["DREI"] = "drei";
            Console.WriteLine(f[3] + "\n" + f[4]);
            
            Console.ReadKey();
        }
    }
}

Als Typ gehen mindestens int, enum und string.