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| 24 |
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| 25 | <div id="content">
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| 26 |
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| 27 | <h2>ReusableConstraint (NUnit 2.5.6)</h2>
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| 28 |
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| 29 | <p>Normally constraints just work. However, attempting to reuse the
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| 30 | same constraint in several places can lead to unexpected results.
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| 31 |
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| 32 | <p>Consider the following code as an example:
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| 33 |
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| 34 | <code><pre>
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| 35 | Constraint myConstraint = Is.Not.Null;
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| 36 | Assert.That("not a null", myConstraint); // Passes, of course
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| 37 | Assert.That("not a null", myConstraint); // Fails! What's that about?
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| 38 | </pre></code>
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| 39 |
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| 40 | <p>We'll save the technical explanation for later and show the
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| 41 | solution first:
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| 42 |
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| 43 | <code><pre>
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| 44 | ReusableConstraint myConstraint = Is.Not.Null;
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| 45 | Assert.That("not a null", myConstraint); // Passes
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| 46 | Assert.That("not a null", myConstraint); // Passes
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| 47 | </pre></code>
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| 48 |
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| 49 | Or alternatively..
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| 50 |
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| 51 | <code><pre>
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| 52 | var myConstraint = new ReusableConstraint(Is.Not.Null);
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| 53 | Assert.That("not a null", myConstraint); // Passes
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| 54 | Assert.That("not a null", myConstraint); // Passes
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| 55 | </pre></code>
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| 56 |
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| 57 | <h3>Technical Explanation</h3>
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| 58 |
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| 59 | <p>In the original example, the value assigned to myConstraint is
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| 60 | known as an <b>unresolved</b> constraint. In fact, it's an
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| 61 | unresolved NullConstraint, because that was the last constraint
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| 62 | encountered in the expression. It's associated with a <b>Not</b>
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| 63 | operator that has not yet been applied.
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| 64 |
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| 65 | <p>That's OK for use with Assert.That(), because the method
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| 66 | knows how to resolve a constraint before using it. Assert.That()
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| 67 | resolves this constraint to a NotConstraint referencing the
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| 68 | original NullConstraint.
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| 69 |
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| 70 | <p>Of course, the original reference in myConstraint is left
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| 71 | unchanged in all of this. But the EqualConstraint it points
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| 72 | to has now been resolved. It is now a <b>resolved</b> constraint
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| 73 | and can't be resolved again by the second Assert.That(), which
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| 74 | only sees the NullConstraint and not the NotConstraint.
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| 75 |
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| 76 | <p>So, for reusability, what we want to save is the result
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| 77 | of resolving the constraint, in this case
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| 78 |
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| 79 | <pre> NotConstraint => NullConstraint</pre>
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| 80 |
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| 81 | That's what <b>ReusableConstraint</b> does for us. It resolves
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| 82 | the full expression and saves the result. Then it passes all
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| 83 | operations on to that saved result.
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| 84 |
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| 85 | <h3>When to Use It</h3>
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| 86 |
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| 87 | <p>Use this constraint any time you want to reuse a constraint
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| 88 | expression and you'll be safe.
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| 89 |
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| 90 | <p>If you like to take chances, you'll find that you can
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| 91 | avoid using it in the following cases...
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| 92 |
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| 93 | <ol>
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| 94 | <li> With a simple constraint involving no operators, like...
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| 95 |
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| 96 | <pre>
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| 97 | Constraint myConstraint = Is.Null;
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| 98 | Constraint myConstraint = Is.EqualTo(42);
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| 99 | </pre>
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| 100 |
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| 101 | <li> With any constraint you construct using new, without
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| 102 | using the "dotted" constraint syntax...
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| 103 |
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| 104 | <pre>
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| 105 | Constraint myConstraint = new NotConstraint(new NullConstraint());
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| 106 | Constraint myConstraint = new AndConstraint(
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| 107 | new GreaterThanConstraint(0),
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| 108 | new LessThanConstraint(100));
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| 109 | </pre>
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| 110 |
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| 111 | <p>However, there is no significant penalty to using <b>ReusableConstraint</b>.
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| 112 | It makes your intent much clearer and the exceptions listed are accidents of
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| 113 | the internal implementation and could disappear in future releases.
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| 114 |
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| 115 |
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| 116 | </div>
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| 117 |
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| 118 | <!-- Submenu -->
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| 119 | <div id="subnav">
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| 120 | <ul>
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| 121 | <li><a href="index.html">NUnit 2.5.10</a></li>
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| 122 | <ul>
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| 123 | <li><a href="getStarted.html">Getting Started</a></li>
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| 124 | <li><a href="assertions.html">Assertions</a></li>
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| 125 | <li><a href="constraintModel.html">Constraints</a></li>
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| 126 | <ul>
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| 127 | <li><a href="equalConstraint.html">Equal Constraint</a></li>
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| 128 | <li><a href="sameasConstraint.html">SameAs Constraint</a></li>
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| 129 | <li><a href="conditionConstraints.html">Condition Constraints</a></li>
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| 130 | <li><a href="comparisonConstraints.html">Comparison Constrants</a></li>
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| 131 | <li><a href="pathConstraints.html">Path Constraints</a></li>
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| 132 | <li><a href="typeConstraints.html">Type Constraints</a></li>
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| 133 | <li><a href="stringConstraints.html">String Constraints</a></li>
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| 134 | <li><a href="collectionConstraints.html">Collection Constraints</a></li>
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| 135 | <li><a href="propertyConstraint.html">Property Constraint</a></li>
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| 136 | <li><a href="throwsConstraint.html">Throws Constraint</a></li>
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| 137 | <li><a href="compoundConstraints.html">Compound Constraints</a></li>
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| 138 | <li><a href="delayedConstraint.html">Delayed Constraint</a></li>
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| 139 | <li><a href="listMapper.html">List Mapper</a></li>
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| 140 | <li id="current"><a href="reusableConstraint.html">Reusable Constraint</a></li>
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| 141 | </ul>
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