113 lines
3.6 KiB
HTML
113 lines
3.6 KiB
HTML
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<h2>Custom Constraints (NUnit 2.4 / 2.5)</h2>
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<p>You can implement your own custom constraints by creating a class that
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inherits from the <b>Constraint</b> abstract class, which supports performing a
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test on an actual value and generating appropriate messages. The class includes
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two abstract methods, which you must override and four virtual methods with
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default implementation that may be overridden as needed:
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<div class="code" style="width: 36em">
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<pre>public abstract class Constraint
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{
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...
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public abstract bool Matches( object actual );
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public virtual bool Matches( ActualValueDelegate del );
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public virtual bool Matches<T>( ref T actual );
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public abstract void WriteDescriptionTo( MessageWriter writer );
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public virtual void WriteMessageTo( MessageWriter writer );
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public virtual void WriteActualValueTo( MessageWriter writer );
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...
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}</pre>
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</div>
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<p>Your derived class should save the actual argument to Matches in the protected
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field actual for later use.
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<p>The MessageWriter abstract class is implemented in the framework by
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TextMessageWriter. Examining the source for some of the builtin constraints
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should give you a good idea of how to use it if you have special formatting
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requirements for error messages.
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<h3>Custom Constraint Syntax</h3>
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<p>NUnit includes classes that implement a special constraint syntax,
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allowing you to write things like...
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<div class="code">
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<pre>Assert.That( myArray, Is.All.InRange(1,100) );</pre>
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</div>
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<p>Of course, the NUnit constraint syntax will not be aware of your
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custom constraint unless you modify NUnit itself. As an alternative,
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you may use the <b>Matches(Constraint)</b> syntactic element in order
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to write code like...
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<div class="code">
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<pre>MyConstraint myConstraint = new MyConstraint();
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Assert.That( myArray, Has.Some.Matches(myConstraint) );</pre>
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