title: extend a class to conform to an interface?
body: <p>I have an interface:</p>
  
  <pre><code>interface IFoo {
    int foo(int bar);
  }
  </code></pre>
  
  <p>Can I now extend an existing class to conform to the interface?  Say class String.  I know I can define the foo() method on strings.  But is it possible to go further and tell the compiler that strings can be cast to IFoo?</p>