Have you tried Decimal.Parse
with specifying decimal separator?
var yourString = "3942.000000000000";
var info = new NumberFormatInfo { NumberDecimalSeparator = "." };
var parsed = Decimal.Parse(yourString, info);
EDIT: as Jon suggested you can use InvariantCulture:
var yourString = "3942.000000000000";
var parsed = Decimal.Parse(yourString, CultureInfo.InvariantCulture);