2 Why We Need Model-Based Testing

Systems with Finite Models 71
action method. Before calling each action method, the test method calls NUnit’s
IsTrue assertion to check the enabling condition. These checks ensure that the run
will succeed only if it executes a sequence of actions that is allowed by the model
program. If it attempts to execute a forbidden sequence, it will fail.
The test method shown in Figure 5.6 programs a run that the model program
should be able to execute. It encodes this sequence of actions:
ShowTitles(); SortByFirst(); SortByMostRecent(); ShowText();
This run succeeds. It produces this output:
Included categories: AllowedRun
.Topics, WithText, ByMostRecent: ShowTitles
Topics, TitlesOnly, ByMostRecent: SortByFirst
Topics, TitlesOnly, ByFirst: SortByMostRecent
Topics, TitlesOnly, ByMostRecent: ShowText
Tests run: 1, Failures: 0, Not run: 0, Time: 0.080 seconds
We also code a run that the model program should not be able to execute:
ShowTitles(); SortByFirst(); SortByFirst(); ShowText();
The model program should not be able to execute this run because there are two
SortByFirst actions in succession. The second of these is not enabled (in the
implementation, this option is not available in this state). The run fails, confirming
that the model program cannot execute this run:
Included categories: ForbiddenRun
.Topics, WithText, ByMostRecent: ShowTitles
Topics, TitlesOnly, ByMostRecent: SortByFirst
F
Tests run: 1, Failures: 1, Not run: 0, Time: 0.080 seconds
Failures:
1) NewsReader.Tests.ForbiddenRun :
at NewsReader.Tests.ForbiddenRun()
This test failure does not indicate that the model program is incorrect. We are
not testing the model program here; we are executing the model program in order
to simulate the implemention. We are only using the test tool because it is more
convenient to write two tests than to write two applications. The failure here indicates
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