<p>I am new to ActionScript 3 and have run into a problem:</p>

<p>Using Flex Builder 3, I have a created a project with a few simple classes. If code in class A instantiates an object of class B (class B is in its own source file) then the code compiles fine, but I get the following run time error:</p>

<blockquote>
  <p>ArgumentError: Error #2012: B class cannot be instantiated.</p>
</blockquote>

<p>Can someone explain what I'm doing wrong?</p>

<p><strong>Update:</strong> Please see my own answer below (I could not vote it to the top since I'm not yet registered).</p>

This usually means that the class information was not included in the SWF.
Make sure that you are importing the class, and that there is a reference to it somewhere (so the compiler will included it in the SWF).
btw, here are the runtime error codes:
http://livedocs.adobe.com/flex/201/langref/runtimeErrors.html
(not much useful info though)
mike chambers
mesh@adobe.com