2 Why We Need Model-Based Testing

Systems with Complex State 157
limited interface. The model program allows you to get the initial state and produce
transitions that take you to subsequent states. Like an enumerator, you can’t find out
how many states a model program has without exploring it step by step.
Model programs with large numbers of states and transitions are not just a
theoretical curiosity. Using values with large domains like strings and integers turns
out to be very useful for modeling real world systems. Data types like int and string
provide descriptive power. We’ll see this with an example later in this chapter. For
now, let’s review the kinds of data types that can be used in writing model programs.
10.2 Types for model programs
You may only use certain types for state variables and action parameters in model
programs that work with the tools we describe. 1 This is necessary because exploration
using these tools can only determine whether two states are equal when the
state variables belong to these types.
Any of the following .NET-provided types may be used for state variables and
action parameters: bool, byte, char, double, float, int, long, short, string as well
as any enum type defined by the model program.
In addition to these built-in types, you may also use the structured types and
collection types described in the rest of this chapter and the object types that we will
introduce later in Chapter 15. 2
10.3 Compound values
Extending our model program with strings and integers doesn’t change the way we
do state comparison. Just as in the previous chapters, two states are equal if their
variables contain the same values. The string data type is compatible with this kind
of equality: two strings are equal if they contain the same characters. Whether the
two strings share the same location in memory doesn’t matter when asking if they
are equal. This kind of equality is called structural equality. It is the kind of equality
that matters for state comparison.
1 For the purposes of this chapter we use the term model program to mean a model program
written in C# source, compiled into a .NET assembly and loaded as a LibraryModelProgram
data type. The NModel package allows extenders to implement other kinds of model programs
that need not rely on .NET types for internal state.
2 The modeling framework provides an interface called IAbstractValue for extending the range
of types permitted in model programs. This is documented in the NModel package. Most users
will not need to do this kind of extension. Creating subclasses of CompoundValue is the easiest
way to add data types.
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