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<h2>Custom Constraints (NUnit 2.4)</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 two virtual methods which
may be overridden as needed:
<div class="code" style="width: 36em">
<pre>public abstract class Constraint
{
...
public abstract bool Matches( object 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.
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