C#

using System;
using System.Collections.Generic;
 
namespace States
{
    public class StateAbbreviations
    {
        private static readonly Dictionary<string, string> stateAbbreviations = new Dictionary<string, string>();
        private static readonly Dictionary<string, string> stateNames = new Dictionary<string, string>();
 
        static StateAbbreviations()
        {
            stateAbbreviations.Add("ALABAMA", "AL");
            stateAbbreviations.Add("ALASKA", "AK");
            stateAbbreviations.Add("AMERICAN SAMOA", "AS");
            stateAbbreviations.Add("ARIZONA", "AZ");
            stateAbbreviations.Add("ARKANSAS", "AR");
            stateAbbreviations.Add("CALIFORNIA", "CA");
            stateAbbreviations.Add("COLORADO", "CO");
            stateAbbreviations.Add("CONNECTICUT", "CT");
            stateAbbreviations.Add("DELAWARE", "DE");
            stateAbbreviations.Add("DISTRICT OF COLUMBIA", "DC");
            stateAbbreviations.Add("FEDERATED STATES OF MICRONESIA", "FM");
            stateAbbreviations.Add("FLORIDA", "FL");
            stateAbbreviations.Add("GEORGIA", "GA");
            stateAbbreviations.Add("GUAM", "GU");
            stateAbbreviations.Add("HAWAII", "HI");
            stateAbbreviations.Add("IDAHO", "ID");
            stateAbbreviations.Add("ILLINOIS", "IL");
            stateAbbreviations.Add("INDIANA", "IN");
            stateAbbreviations.Add("IOWA", "IA");
            stateAbbreviations.Add("KANSAS", "KS");
            stateAbbreviations.Add("KENTUCKY", "KY");
            stateAbbreviations.Add("LOUISIANA", "LA");
            stateAbbreviations.Add("MAINE", "ME");
            stateAbbreviations.Add("MARSHALL ISLANDS", "MH");
            stateAbbreviations.Add("MARYLAND", "MD");
            stateAbbreviations.Add("MASSACHUSETTS", "MA");
            stateAbbreviations.Add("MICHIGAN", "MI");
            stateAbbreviations.Add("MINNESOTA", "MN");
            stateAbbreviations.Add("MISSISSIPPI", "MS");
            stateAbbreviations.Add("MISSOURI", "MO");
            stateAbbreviations.Add("MONTANA", "MT");
            stateAbbreviations.Add("NEBRASKA", "NE");
            stateAbbreviations.Add("NEVADA", "NV");
            stateAbbreviations.Add("NEW HAMPSHIRE", "NH");
            stateAbbreviations.Add("NEW JERSEY", "NJ");
            stateAbbreviations.Add("NEW MEXICO", "NM");
            stateAbbreviations.Add("NEW YORK", "NY");
            stateAbbreviations.Add("NORTH CAROLINA", "NC");
            stateAbbreviations.Add("NORTH DAKOTA", "ND");
            stateAbbreviations.Add("NORTHERN MARIANA ISLANDS", "MP");
            stateAbbreviations.Add("OHIO", "OH");
            stateAbbreviations.Add("OKLAHOMA", "OK");
            stateAbbreviations.Add("OREGON", "OR");
            stateAbbreviations.Add("PALAU", "PW");
            stateAbbreviations.Add("PENNSYLVANIA", "PA");
            stateAbbreviations.Add("PUERTO RICO", "PR");
            stateAbbreviations.Add("RHODE ISLAND", "RI");
            stateAbbreviations.Add("SOUTH CAROLINA", "SC");
            stateAbbreviations.Add("SOUTH DAKOTA", "SD");
            stateAbbreviations.Add("TENNESSEE", "TN");
            stateAbbreviations.Add("TEXAS", "TX");
            stateAbbreviations.Add("UTAH", "UT");
            stateAbbreviations.Add("VERMONT", "VT");
            stateAbbreviations.Add("VIRGIN ISLANDS", "VI");
            stateAbbreviations.Add("VIRGINIA", "VA");
            stateAbbreviations.Add("WASHINGTON", "WA");
            stateAbbreviations.Add("WEST VIRGINIA", "WV");
            stateAbbreviations.Add("WISCONSIN", "WI");
            stateAbbreviations.Add("WYOMING", "WY");
 
            foreach(KeyValuePair<string, string> pair in stateAbbreviations)
            {
                stateNames.Add(pair.Value, pair.Key);
            }
        }
 
        public static string GetStateName(string stateAbbreviation)
        {
            string name;
 
            if(stateAbbreviation == null)
            {
                throw new ArgumentNullException("State abbreviation cannot be null");
            }
            else 
            {
                if(!stateNames.TryGetValue(stateAbbreviation.ToUpper(), out name))
                {
                    throw new ArgumentException("Unknown abbreviation");
                }
            }
 
            return name;
        }
 
        public static string GetStateAbbreviation(string stateName)
        {
            string abbreviation;
 
            if(stateName == null)
            {
                throw new ArgumentNullException("State name cannot be null");
            }
            else
            {
                if(!stateAbbreviations.TryGetValue(stateName.ToUpper(), out abbreviation))
                {
                    throw new ArgumentException("Unknown state");
                }
            }
 
            return abbreviation;
        }
 
        public static IEnumerable<string> Abbreviations
        {
            get
            {
                return stateAbbreviations.Values;
            }
        }
 
        public static IEnumerable<string> Names
        {
            get
            {
                return stateNames.Values;
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Text;
 
namespace States
{
    class Program
    {
        static void Main(string[] args)
        {
            foreach(string name in StateAbbreviations.Names)
            {
                Console.WriteLine(StateAbbreviations.GetStateAbbreviation(name));
            }
 
            Console.WriteLine();
 
            foreach(string abbreviaton in StateAbbreviations.Abbreviations)
            {
                Console.WriteLine(StateAbbreviations.GetStateName(abbreviaton));
            }
 
            Console.Read();
        }
    }
}

code-frog wrote:

1. Your implementation was/is more elegant than mine. I did it with an ArrayList.
2. Yours is complete. Mine is still being written because the phone keeps ringing.

So because I'm a highly evolved being, short on time and know a good thing when I see it. I'll use yours... at my own risk of course.

Thanks a ton! Now that's probably the most helpful reply I have ever received in my life.