Hi Sean,

I think in this case I would suggest you do the conversion in the payment provider as allowing the stored code to be changeable could have adverse effects on code that expects it in the 2 letter format (ie, I think consistency is more beneficial than flexibility here).

With that being the case you have 2 options really. Store a hard coded lookup table of 2 to 3 letter ISO3166 codes. Or potentially look at using CultureInfo to do it dynamically

public string ConvertTwoLetterNameToThreeLetterName(string name)
{
    if (name.Length != 2)
    {
        throw new ArgumentException("name must be two letters.");
    }

    name = name.ToUpper();

    var cultures = CultureInfo.GetCultures(CultureTypes.SpecificCultures);
    foreach (var culture in cultures)
    {
        var region = new RegionInfo(culture.LCID);
        if (region.TwoLetterISORegionName.ToUpper() == name)
        {
            return region.ThreeLetterISORegionName;
        }
    }

    return null;
}

This is modified from a stack overflow post which has some good other alternatives too https://stackoverflow.com/questions/4884692/converting-country-codes-in-net

/Matt