The Single Choice principle:
 whenever a software system must support a set of variants, clients using objects from this set should not have knowledge of all the elements in the set. A violation of this principle can be illustrated with the variants of Pascal or the unions of C (as long as the union is placed inside a struct and there is a field that acts as a type descriptor). Code written that acts upon one of these “multiple personality entities”, must be queried for its actual type, and we must program accordingly. This will produce multiple selection constructs in any module that uses these variant types.

Such constructs will become a hindrance to software change and evolution. Any addition of a new type to the variant set will force modifications in all modules that have to query the type to act accordingly. This multiple choices spread around many modules of a system is exactly the opposite to the single choice principle.

Traditional languages and paradigms do not provide a solution. On the other hand, object orientation, via polymorphism and dynamic binding present an elegant and complete implementation of this principle.

This concludes our consideration of the properties necessary for good modular structures. It will be seen that Abstract Data Types provide an adequate specification for modules that are consistent with the properties developed in this section. Eventually, we will cross the bridge into the software world and introduce the class as a potentially partial implementation of an Abstract Data Type.