162 lines
5.5 KiB
HTML
162 lines
5.5 KiB
HTML
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<h2>ReusableConstraint (NUnit 2.5.6)</h2>
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<p>Normally constraints just work. However, attempting to reuse the
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same constraint in several places can lead to unexpected results.
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<p>Consider the following code as an example:
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<code><pre>
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Constraint myConstraint = Is.Not.Null;
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Assert.That("not a null", myConstraint); // Passes, of course
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Assert.That("not a null", myConstraint); // Fails! What's that about?
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</pre></code>
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<p>We'll save the technical explanation for later and show the
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solution first:
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<code><pre>
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ReusableConstraint myConstraint = Is.Not.Null;
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Assert.That("not a null", myConstraint); // Passes
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Assert.That("not a null", myConstraint); // Passes
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</pre></code>
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Or alternatively..
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<code><pre>
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var myConstraint = new ReusableConstraint(Is.Not.Null);
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Assert.That("not a null", myConstraint); // Passes
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Assert.That("not a null", myConstraint); // Passes
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</pre></code>
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<h3>Technical Explanation</h3>
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<p>In the original example, the value assigned to myConstraint is
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known as an <b>unresolved</b> constraint. In fact, it's an
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unresolved NullConstraint, because that was the last constraint
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encountered in the expression. It's associated with a <b>Not</b>
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operator that has not yet been applied.
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<p>That's OK for use with Assert.That(), because the method
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knows how to resolve a constraint before using it. Assert.That()
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resolves this constraint to a NotConstraint referencing the
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original NullConstraint.
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<p>Of course, the original reference in myConstraint is left
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unchanged in all of this. But the EqualConstraint it points
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to has now been resolved. It is now a <b>resolved</b> constraint
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and can't be resolved again by the second Assert.That(), which
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only sees the NullConstraint and not the NotConstraint.
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<p>So, for reusability, what we want to save is the result
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of resolving the constraint, in this case
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<pre> NotConstraint => NullConstraint</pre>
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That's what <b>ReusableConstraint</b> does for us. It resolves
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the full expression and saves the result. Then it passes all
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operations on to that saved result.
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<h3>When to Use It</h3>
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<p>Use this constraint any time you want to reuse a constraint
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expression and you'll be safe.
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<p>If you like to take chances, you'll find that you can
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avoid using it in the following cases...
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<ol>
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<li> With a simple constraint involving no operators, like...
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<pre>
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Constraint myConstraint = Is.Null;
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Constraint myConstraint = Is.EqualTo(42);
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</pre>
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<li> With any constraint you construct using new, without
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using the "dotted" constraint syntax...
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<pre>
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Constraint myConstraint = new NotConstraint(new NullConstraint());
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Constraint myConstraint = new AndConstraint(
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new GreaterThanConstraint(0),
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new LessThanConstraint(100));
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</pre>
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<p>However, there is no significant penalty to using <b>ReusableConstraint</b>.
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It makes your intent much clearer and the exceptions listed are accidents of
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the internal implementation and could disappear in future releases.
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<li><a href="index.html">NUnit 2.5.9</a></li>
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