2 Why We Need Model-Based Testing

Systems with Complex State 187
We might want to consider only states where the counter is 0, 1, or2. Clearly,
this is a small subset of the possible states for this model program, but it is a good
place to start when trying to analyze the behavior of the system.
To do this we can use a state filter. A state filter is a Boolean condition that must
be satisfied in the end state of a transition in order for the transition to be included
in the exploration. State filters say which states are “interesting” for analysis.
We can keep the contract separate from the “small state” scenario limitation by
putting the state filter in a feature:
namespace Counter2
{
[Feature]
static class SmallState
{
[StateFilter]
static bool WithinCounterLimit()
{ return Contract.counter < 3; }
}
}
If you include the SmallState feature in your model program factory, then exploration
via the mpv tool will include only transitions that result in a state in which
the WithinCounterLimit method returns true.
11.2.3 Strenghening enabling conditions
Sometimes it is useful to omit certain kinds of transitions that are allowed by the
contract model program but are of less interest for a test purpose or analysis goal.
A general technique that can be used here is to strengthen enabling conditions.
With this technique, you can add additional constraints that depend on the state of
the system and the action parameters.
namespace Counter2
{
static class PreconditionLimits
{
static bool IncrementEnabled()
{ return Contract.counter < 2; }
}
}
[Action]
static void Increment() { }
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