2 Why We Need Model-Based Testing

252 Modeling Objects
}
}
void SetSalary(int x)
{ this.salary = x; }
public override void Initialize()
{ this.salary = 0; }
If you compare this version to the previous example, you will notice that we have
introduced a C# instance field for salary. Also, instance methods have replaced some
of the static methods for the actions.
The method of instance creation differs as well. The Domain attribute that supplies
parameter values for the CreateEmployee is given the string "new" as its argument.
The NModel framework provides a parameter generator function for each class that
inherits LabeledInstance.
Both versions of this example have exactly the same possible traces. For example,
Payroll1 and Payroll2 may both have the trace:
CreateEmployee(Employee(1))
SetSalary(Employee(1), 200)
CreateEmployee(Employee(2))
SetSalary(Employee(2), 400)
DeleteEmployee(Employee(1))
If you were to inspect the state, you would see that the modeling tool maintains
a field map for you for the salary field. It also maintains state for the next object ID
to be allocated.
The keys of a field map created for an instance field contain all reachable instances
of the class. At the end of the sample trace shown above, there is just one reachable
object, Employee(2). An action like DeleteEmployee(Employee(1)) can eliminate
an element in the salary field map because when Employee(1) is removed from the
allEmployees field, it becomes unreachable.
Note that actions of the model program may be instance methods. In this case the
this parameter becomes the first parameter of the action. Appendix A.1.3 describes
in more detail how the parameters of C# methods correspond to the parameters of
actions of the model program.
Also, the action vocabulary of a model program consists of distinct symbols.
Overloading is not supported for action names. Action names should be chosen so
that they are easy to understand and unambiguous. For example, we used CreateEmployee
rather than the shorter name Create that would more conflict with action
names associated with other data types.
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