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);
            }
        }
    }

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