title: Can't get a reference to IPart
body: <p>I'm trying to get a reference to the IPart interface to control Windows audio volume level, for example, and I'm using the <code>IDeviceTopology::GetPartById</code> function to get it.</p>
  
  <p>This function takes a UINT. I tried 0 and got nothing, then gave in and just tried incrementing a UINT until I could get a reference. No luck that way.</p>
  
  <p>I've got a reference to the correct device topology.</p>
  
  <p>My HRESULT is <code>E_INVALIDARG</code>. </p>
  
  <p>Is it just the case that there aren't any parts available on my device?</p>
  
  <p><a href="http://msdn.microsoft.com/en-us/library/dd368220%28v=VS.85%29.aspx" rel="nofollow">MSDN</a> says to get an ID to pass into GetPartById, I should call <code>IAudioInputSelector::GetSelection</code>. To use IAudioInputSelector though, I need to already have a reference to an IPart object.</p>