title: Asynchronous call of inprocess COM-DLL from VSTO Excel Addin?
body: <p>I am developing an application level VSTO 4 Addin for Microsoft Excel 2007 / 2010. 
  The result is a windows forms based DLL using .Net 4 Client Profile.</p>
  
  <p>Now I have to use a legacy COM-DLL. It is no problem to set the reference and access the COM-Methods via COM-Interop from .Net.</p>
  
  <p>But the the (synchronous) method I need to call can take a minute or longer to get back.</p>
  
  <p>I know your answer: 
  Use a worker thread... </p>
  
  <p>I <strong>have</strong> used The Task Parallel Library to put the long lasting operation in a worker task and keep the GUI (Excel) responding.</p>
  
  <hr>
  
  <p>But: The inprocess COM-Call (in the worker task/thread) still seems to <strong>block</strong> my GUI-Thread.</p>
  
  <ul>
  <li>Why? Is it because Excel is always running as STA (Single Thread
  Apartment)?</li>
  <li>How can I keep the Excel GUI responding? </li>
  <li>Is there a way to make it really asynchronous?</li>
  </ul>
  
  <hr>
  
  <p>Thanks for any answers,</p>
  
  <p>Jörg</p>