public void DoSomething()
 {
     using (Thingamabob t = new Thingamabob()) {
         Bar b = new Bar();
         b.Foo(t);
     }
 }
 public static void Foo(this Bar b, Thingamabob t)
 {
     // Access t to enable this extension method to do its work, whatever that may be
 }