public class IBar
{
  void DoBar();
  void DoBaz();
  event EventHandler BarChanged;
}

public abstract class BaseBar : IBar {
   ... implementation ...
   public abstract void DoGaz();
   public virtual void DoFoo() {...}
}

public class ABar : BaseBar
{
  ... implementation ...
}

public class BBar : BaseBar
{

  ... implementation ...
}

public static class BarFactory
{
   public static IBar MakeBar(string arg) {
        switch(arg) {
           case "xxx":
             return new ABar();
           case "qqq":
             return new BBar();
           ... and so on ...
        }
   }

}