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copied!<p>I just finished creating something very similar for a system I'm working on. The theory, in general, is to separate the definition (who they are) of an entity from its behaviour (roles; what they do). I also wanted to preserve the ability to query users based on the metadata contained in the roles that they are assigned to.</p> <p>I ended up creating a <code>Person</code> class that looks like this:</p> <pre><code>class Person { Guid Id; ISet<Role> Roles; string Name; // and any other stuff a "person" might have } </code></pre> <p>Next, I have an abstract <code>Role</code> class that looks like this:</p> <pre><code>abstract class Role { Guid Id; abstract string Name { get; } // implemented by concrete definitions of roles Person Person; } </code></pre> <p>From this, I derive individual classes that "define" each role I have in my system. A <code>User</code> role would look something like this:</p> <pre><code>class User : Role { override string Name { get { return "User"; } } string LoginId; string Password; // etc. } </code></pre> <p>Rinse and repeat for any other roles you might have.</p> <p>At the ORM level (where I use NHibernate), I define UserRole as a joined-subclass of the Role entity.</p> <p>The underlying assumptions behind this approach are:</p> <ul> <li>Roles aren't dynamic.</li> </ul> <p>There is no definition of roles at the data layer. A role exists simply because there's a class that derives from it.</p> <ul> <li>You can't assign a role to the same person multiple times with different metadata</li> </ul> <p>Well, in theory, I guess you could but it makes no sense for my particular system for a person to have two <code>User</code> roles assigned to them with different login ids and passwords.</p>

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