1. A character string data processing method for passing a first character string from a first object, through an interface object running under control of a computer operating system, to a second object when an application program is run, the method comprising: casting, in the interface object, the first character string data as basic string data whereby a number of characters including null data in the character string data is countable irrespective of a declared type of the first character string data of the first object, detecting an effective number of characters in the first character string data as a basic string, and converting the effective number of characters in the first character string to character string data that can be passed by the operating system; and processing, in the second object, the first character string data as basic string data irrespective of the declared type of the first character string data.