rt class was used for each
   different assertion. We call this the "Classic Model." It
   continues to be supported in NUnit, since many people prefer it.</p>

<p>Beginning with NUnit 2.4, a new "Constraint-based" model was 
   introduced. This approach uses a single method of the Assert class
   for all assertions, passing a <a href="constraintModel.html">Constraint</a>   object that specifies the test to be performed.

<p>This constraint-based model is now used internally by NUnit
   for all assertions. 