2 Why We Need Model-Based Testing

Systems with Complex State 199
The DoAction method makes a step according to the given input action. It may
return either an output action or the special value null, which indicates that there is
no immediate output action. You should implement the DoAction method to throw an
exception if the input action fails. The Reset method should reset the implementation
to its initial state. If the stepper cannot reset the implementation from its current
state, it should throw an exception.
Since we are not interested in testing multiple instances of BagImpl, we assume
that the initial state of the stepper references a fixed instance of BagImpl that has
been created initially.
These are the action symbols that the stepper must handle:
• An Add symbol that takes a string as an argument.
• A Delete symbol that takes a string as an argument.
• A Lookup Start symbol that takes a string as an argument, and a Lookup Finish
symbol that takes an integer as an argument. The action Lookup Start(e) represents
the invocation of the bag implementation’s Lookup method with input
parameter e, and the action Lookup Finish(k) represents the return value k of the
invocation.
• A Count Start symbol and a Count Finish symbol that take an integer as an argument.
The action Count Start() represents the invocation of the bag implementation’s
implicit get Count method, and the action Count Finish(k) represents
the return value k of the invocation.
A stepper for BagImpl is shown in Figure 12.4. In order to map the stepper’s input
actions to concrete inputs for the bag implementation, the stepper must extract the
first argument of the input action a, using the indexer expression a[0] and a cast
to string. The modeling library provides action arguments through a C# indexer,
which accesses elements by subscripts. In order to construct the output actions that
correspond to the return values of the lookup and count operations, the stepper uses
the utility function Action.Create to create an output action from a given action
name and arguments given as .NET values. (Appendix A.3 explains these and other
facilities for working with action terms.)
12.3 Strategies
In a typical model, there are multiple possible actions enabled in any given model
state. With offline test generation, the main idea is that the model is analyzed globally
and a test suite is derived that provides a traversal of the model.
The offline case is a special case of a more general form of testing where the
tester has a strategy selecting the action to be executed next or what result is to be
expected from the system under test. In addition to the model state, a tester also has
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