C# convert two letter country codes to three letter country codes

CountryCodeConverter.cs

    public static class CountryCodeConverter
    {
        public static string ConvertThreeLetterNameToTwoLetterName(string twoLetterCountryCode)
        {
            if (twoLetterCountryCode == null || twoLetterCountryCode.Length != 2)
            {
                throw new ArgumentException("name must be three letters.");
            }

            CultureInfo[] cultures = CultureInfo.GetCultures(CultureTypes.SpecificCultures);

            foreach (CultureInfo culture in cultures)
            {
                RegionInfo region = new RegionInfo(culture.LCID);
                if (region.TwoLetterISORegionName.ToUpper() == twoLetterCountryCode.ToUpper())
                {
                    return region.ThreeLetterISORegionName;
                }
            }

            throw new ArgumentException("Could not get country code");
        }
    }

CountryCodeConverterTests.cs

  [TestClass]
    public class CountryCodeConverterTest
    {
        [TestMethod]
        public void Test_All_Country_Code_Conversion()
        {
            var countryCodes = new Dictionary<string, string>() {
               { "AFG", "AF" },    // Afghanistan
               { "ALB", "AL" },    // Albania
               { "ARE", "AE" },    // U.A.E.
               { "ARG", "AR" },    // Argentina
               { "ARM", "AM" },    // Armenia
               { "AUS", "AU" },    // Australia
               { "AUT", "AT" },    // Austria
               { "AZE", "AZ" },    // Azerbaijan
               { "BEL", "BE" },    // Belgium
               { "BGD", "BD" },    // Bangladesh
               { "BGR", "BG" },    // Bulgaria
               { "BHR", "BH" },    // Bahrain
               { "BIH", "BA" },    // Bosnia and Herzegovina
               { "BLR", "BY" },    // Belarus
               { "BLZ", "BZ" },    // Belize
               { "BOL", "BO" },    // Bolivia
               { "BRA", "BR" },    // Brazil
               { "BRN", "BN" },    // Brunei Darussalam
               { "CAN", "CA" },    // Canada
               { "CHE", "CH" },    // Switzerland
               { "CHL", "CL" },    // Chile
               { "CHN", "CN" },    // People's Republic of China
               { "COL", "CO" },    // Colombia
               { "CRI", "CR" },    // Costa Rica
               { "CZE", "CZ" },    // Czech Republic
               { "DEU", "DE" },    // Germany
               { "DNK", "DK" },    // Denmark
               { "DOM", "DO" },    // Dominican Republic
               { "DZA", "DZ" },    // Algeria
               { "ECU", "EC" },    // Ecuador
               { "EGY", "EG" },    // Egypt
               { "ESP", "ES" },    // Spain
               { "EST", "EE" },    // Estonia
               { "ETH", "ET" },    // Ethiopia
               { "FIN", "FI" },    // Finland
               { "FRA", "FR" },    // France
               { "FRO", "FO" },    // Faroe Islands
               { "GBR", "GB" },    // United Kingdom
               { "GEO", "GE" },    // Georgia
               { "GRC", "GR" },    // Greece
               { "GRL", "GL" },    // Greenland
               { "GTM", "GT" },    // Guatemala
               { "HKG", "HK" },    // Hong Kong S.A.R.
               { "HND", "HN" },    // Honduras
               { "HRV", "HR" },    // Croatia
               { "HUN", "HU" },    // Hungary
               { "IDN", "ID" },    // Indonesia
               { "IND", "IN" },    // India
               { "IRL", "IE" },    // Ireland
               { "IRN", "IR" },    // Iran
               { "IRQ", "IQ" },    // Iraq
               { "ISL", "IS" },    // Iceland
               { "ISR", "IL" },    // Israel
               { "ITA", "IT" },    // Italy
               { "JAM", "JM" },    // Jamaica
               { "JOR", "JO" },    // Jordan
               { "JPN", "JP" },    // Japan
               { "KAZ", "KZ" },    // Kazakhstan
               { "KEN", "KE" },    // Kenya
               { "KGZ", "KG" },    // Kyrgyzstan
               { "KHM", "KH" },    // Cambodia
               { "KOR", "KR" },    // Korea
               { "KWT", "KW" },    // Kuwait
               { "LAO", "LA" },    // Lao P.D.R.
               { "LBN", "LB" },    // Lebanon
               { "LBY", "LY" },    // Libya
               { "LIE", "LI" },    // Liechtenstein
               { "LKA", "LK" },    // Sri Lanka
               { "LTU", "LT" },    // Lithuania
               { "LUX", "LU" },    // Luxembourg
               { "LVA", "LV" },    // Latvia
               { "MAC", "MO" },    // Macao S.A.R.
               { "MAR", "MA" },    // Morocco
               { "MCO", "MC" },    // Principality of Monaco
               { "MDV", "MV" },    // Maldives
               { "MEX", "MX" },    // Mexico
               { "MKD", "MK" },    // Macedonia (FYROM)
               { "MLT", "MT" },    // Malta
               { "MNE", "ME" },    // Montenegro
               { "MNG", "MN" },    // Mongolia
               { "MYS", "MY" },    // Malaysia
               { "NGA", "NG" },    // Nigeria
               { "NIC", "NI" },    // Nicaragua
               { "NLD", "NL" },    // Netherlands
               { "NOR", "NO" },    // Norway
               { "NPL", "NP" },    // Nepal
               { "NZL", "NZ" },    // New Zealand
               { "OMN", "OM" },    // Oman
               { "PAK", "PK" },    // Islamic Republic of Pakistan
               { "PAN", "PA" },    // Panama
               { "PER", "PE" },    // Peru
               { "PHL", "PH" },    // Republic of the Philippines
               { "POL", "PL" },    // Poland
               { "PRI", "PR" },    // Puerto Rico
               { "PRT", "PT" },    // Portugal
               { "PRY", "PY" },    // Paraguay
               { "QAT", "QA" },    // Qatar
               { "ROU", "RO" },    // Romania
               { "RUS", "RU" },    // Russia
               { "RWA", "RW" },    // Rwanda
               { "SAU", "SA" },    // Saudi Arabia
               { "SCG", "CS" },    // Serbia and Montenegro (Former)
               { "SEN", "SN" },    // Senegal
               { "SGP", "SG" },    // Singapore
               { "SLV", "SV" },    // El Salvador
               { "SRB", "RS" },    // Serbia
               { "SVK", "SK" },    // Slovakia
               { "SVN", "SI" },    // Slovenia
               { "SWE", "SE" },    // Sweden
               { "SYR", "SY" },    // Syria
               { "TAJ", "TJ" },    // Tajikistan
               { "THA", "TH" },    // Thailand
               { "TKM", "TM" },    // Turkmenistan
               { "TTO", "TT" },    // Trinidad and Tobago
               { "TUN", "TN" },    // Tunisia
               { "TUR", "TR" },    // Turkey
               { "TWN", "TW" },    // Taiwan
               { "UKR", "UA" },    // Ukraine
               { "URY", "UY" },    // Uruguay
               { "USA", "US" },    // United States
               { "UZB", "UZ" },    // Uzbekistan
               { "VEN", "VE" },    // Bolivarian Republic of Venezuela
               { "VNM", "VN" },    // Vietnam
               { "YEM", "YE" },    // Yemen
               { "ZAF", "ZA" },    // South Africa
               { "ZWE", "ZW" },    // Zimbabwe
            };

            foreach (var cnt in countryCodes)
            {
                Assert.AreEqual(CountryCodeConverter.ConvertThreeLetterNameToTwoLetterName(cnt.Value), cnt.Key);
            }
        }
    }

Hi Ben, thank you for this! I wanted to notify you of an issue on line 14 of CountryCodeConverter.cs - it seems that many cultures have the same cultureID or culture.LCID so you won't get the right result in certain cases. "Guam" is one of those cases. Switching that line to use the culture.Name instead will correct the issue.

Line 14:
RegionInfo region = new RegionInfo(culture.Name);

Nice comment @mombrea it's helpful for those that find our way here 👍

For anyone finding their way here looking to map three letter country codes to two letter codes, here's a streamlined adaptation of this gist as a Static Helper class incorporating the fix noted by @mombrea above along with faster lookup caching:

https://gist.github.com/cajuncoding/69422463cf74f8162ec25d9149ce91a8