title: How to force sub classes to implement a method
body: <p>I am creating an object structure and I want all sub classes of the base to be forced to implement a method.</p>
  
  <p>The only ways I could think of doing it were:</p>
  
  <ol>
  <li><p>An abstract class - Would work but the base class has some useful helper functions that get used by some of the sub classes.</p></li>
  <li><p>An interface - If applied to just the base class then the sub classes don't have to implement the function only the base class does.</p></li>
  </ol>
  
  <p>Is this even possible?</p>
  
  <p>N.B. This is a .NET 2 app.</p>