2 Why We Need Model-Based Testing

15 Modeling Objects
In the previous chapters we showed how the state variables of a model program
can be expressed as the fields of classes declared in a given C# namespace. Up
to this point in our presentation such fields have all been global, that is, declared
with the C# static keyword. In this chapter, we show how model programs can be
written to use instance-based fields as state variables. This is sometimes called an
object-oriented style of modeling.
We will also show how instances of C# classes (“objects”) be thought of as abstract
identifiers. Abstract values like object IDs may be considered to be an equivalence
class when exploring the model. Ignoring states that are structurally identical to
a previous state except for their choice of object IDs can make exploration more
efficient.
There is nothing fundamental about the object-oriented style of modeling; equivalent
behavior can also be expressed using the structure-based style presented in
Chapter 10. The motivation for objects is primarily convenience. Many programmers
are familiar with the object-oriented style, and writing a model in this style
can sometimes improve its readability.
15.1 Instance variables as field maps
Structures like maps can be used to encode state that occurs “per-instance.” For
example, we could use maps to model a payroll system:
namespace Payroll1
{
class Employee : CompoundValue
{
public readonly int id;
public Employee(int id) { this.id = id; }
}
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