title: How can I cast between interface?
body: <p>there are interface a, b, c;</p>
  
  <p>// defined.
  interface d : b, c;</p>
  
  <p>class C1 : a, b, c;</p>
  
  <p>// I want to cast to interface d;</p>
  
  <pre><code>var t = c1 as d;
  </code></pre>
  
  <p>// t is null.</p>