You can implement your own custom constraints by creating a class that implements the IConstraint interface, which supports performing a test on an actual value and generating appropriate messages. The interface is defined as follows:
public interface IConstraint
{
bool Matches( object actual );
void WriteMessageTo( MessageWriter writer );
void WriteDescriptionTo( MessageWriter writer );
void WriteActualValueTo( MessageWriter writer );
}
One approach is to derive from the abstract class Constraint, which provides the & and | operators as well as a default implementation of the WriteMessageTo() and WriteActualValueTo() methods.
Your derived class should save the actual argument to Matches for later use. If deriving from Constraint, use the protected field actual for this purpose.
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.