public static AreaUnit CreateAreaUnit( 
   string name,
   double conversionFactor
)

try
{
  var myFactoryCodeInit = AreaUnit.CreateAreaUnit(109439);     // 109439 is the factory code for square miles

  var myWktUnit = AreaUnit.CreateAreaUnit("HECTARE_AREAUNIT[\"H\",10000.0]");

  var myCustomUnit = AreaUnit.CreateAreaUnit("myAreaUnit", 12);
}
catch (ArgumentException)
{
  // ArgumentException will be thrown by CreateAreaUnit in the following scenarios
  // - if the factory code used is a non-areal factory code  (i.e. it corresponds to degrees which is an angular unit code)
  // - if the factory code used is invalid (i.e. it is negative or doesn't correspond to any factory code)
}