title: How to Make a Custom Extension for Reactive Extensions
body: <p>It is not hard to find an example of <a href="http://www.codeproject.com/Articles/37373/Writing-Custom-LINQ-Extension-Methods" rel="nofollow">how to make a custom LINQ extension method</a>.  But I can't find an example of how to make a custom Rx extension method.</p>
  
  <p>Can someone point me to a resource or post an example?  I'm using the latest (ver 2.2.5).</p>
  
  <p>I'm interested in making a custom "Cast" or custom "Select".  I can reflect the source code of existing extensions, and it is still non-obvious.</p>
  
  <p>Thanks.</p>