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<h2>Assertions</h2>
<p>Assertions are central to unit testing in any of the xUnit frameworks, and NUnit
is no exception. NUnit provides a rich set of assertions as static methods of
the Assert class.</p>
<p>If an assertion fails, the method call does not return and an error is reported.
If a test contains multiple assertions, any that follow the one that failed
will not be executed. For this reason, it's usually best to try for one
assertion per test.</p>
<p>Each method may be called without a message, with a simple text message or with
a message and arguments. In the last case the message is formatted using the
provided text and arguments.</p>
<h3>Two Models</h3>
<p>Before NUnit 2.4, a separate method of the Assert class was used for each
different assertion. We call this the "Classic Model." It
continues to be supported in NUnit, since many people prefer it.</p>
<p>Beginning with NUnit 2.4, a new "Constraint-based" model was
introduced. This approach uses a single method of the Assert class
for all assertions, passing a <a href="constraintModel.html">Constraint</a> object that specifies the test to be performed.
<p>This constraint-based model is now used internally by NUnit
for all assertions. The methods of the classic approach have been
re-implemented on top of this new model.
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