title: Using WM_USER, WM_APP or RegisterWindowMessage
body: <p>Currently, I have a Windows EXE application, with several loaded DLLs. DLLs need to communicate with my windows application through <code>PostMessage</code> and <code>SendMessage</code>. </p>
  
  <p>The Windows EXE application + DLLs are all within a single process.</p>
  
  <p>The message should be private among EXE and DLLs.</p>
  
  <p>I was wondering, should I use</p>
  
  <pre><code> - WM_USER based message
   - WM_APP based message
   - RegisterWindowMessage
  </code></pre>
  
  <p>and why? </p>
  
  <p>What happen if there is an external process (another exe), trying to FindWindow of my Windows application, and send the message with same ID? </p>
  
  <p>I wish not to respond, as I am only interested message from DLLs within my own process.</p>