C#

private object foo = null;

public object Foo
{
  get { if (foo == null) foo = new object(); return foo; }
}

public interface IFoo
{
  void Test();
}
class Foo1 : IFoo
{
  void Test() { Console.WriteLine("from Foo1"); }
}
class Foo2 : IFoo
{
  void Test() { Console.WriteLine("from Foo2"); }
}
public class FooFactory
{
  public static IFoo CreateFoo(string cls)
  {
    if (cls == "foo1") return new Foo1();
    else if (cls == "foo2") return new Foo2();
    else return null;
  }
}

<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <probing privatePath="plugins"/>
    </assemblyBinding>
  </runtime>
</configuration>