title: #include <gdiplus.h> causes error
body: <p>When I include <code>gdiplus.h</code> in a program that compiles well the first(there are many) error I get is:</p>
  
  <pre><code>c:\program files (x86)\microsoft sdks\windows\v7.0a\include\GdiplusImaging.h(77): error C2504: 'IUnknown' : base class undefined
  </code></pre>
  
  <p>Part of GdiplusImaging.h:</p>
  
  <pre><code>IImageBytes : public IUnknown  &lt;&lt;&lt; error!
  {
  public:
       ...
  </code></pre>
  
  <p>Why it is so? Where is this IUnknown class? And why it's not in GdiplusImaging.h?</p>
  
  <p>My system is Windows7 x64. VisualStudio 2010.</p>
  
  <p>Including part:</p>
  
  <pre><code>#include &lt;windows.h&gt;
  #include &lt;gdiplus.h&gt;
  using namespace Gdiplus;
  #pragma comment (lib, "gdiplus.lib")
  </code></pre>