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