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    <p>When you need an in-proc COM object, you don't link directly to the implementing DLL. You usually use <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms686615%28v=vs.85%29.aspx" rel="nofollow"><code>CoCreateInstance</code></a>/<a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms680701%28v=vs.85%29.aspx" rel="nofollow"><code>CoCreateInstanceEx</code></a>, which will load the DLL for you.</p> <p>The lookup goes through the <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa374191%28v=vs.85%29.aspx" rel="nofollow">application's manifest</a> and its dependant <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa374219%28v=vs.85%29.aspx" rel="nofollow">assembly manifests</a>. This is done to support <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa375134%28v=vs.85%29.aspx" rel="nofollow">registration-free COM</a>.</p> <p>If there's no application manifest or if none of the dependant assembly manifests declare your class in a <code>comClass</code> XML element, the lookup defaults to the registry, which will check <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms691424%28v=vs.85%29.aspx" rel="nofollow"><code>HKEY_CLASSES_ROOT\CLSID</code></a><sup>1</sup> for a subkey named <code>{&lt;your-CLSID&gt;}</code>, itself with an <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms682390%28v=vs.85%29.aspx" rel="nofollow"><code>InProcServer32</code></a> subkey stating the DLL.</p> <p>This explains why SomeCOMlib.dll doesn't appear as a dependency. It doesn't explain why LibA.dll doesn't appear as a dependency of it, probably because it's dynamically loaded. If you profile your app within Dependency Walker, you'll see a log of LoadLibrary calls in the bottom pane. To profile it, open your Python executable in Dependency Walker, then go to menu option Profile->Start profiling..., set the parameters to run your <code>.py</code> file and click Ok.</p> <p>The 0xE06D7363 exception code is a <a href="http://support.microsoft.com/kb/185294" rel="nofollow">Visual C++ exception code</a>. You should check the source code of <code>doSomethingWithCOMobj</code>. To debug it, use your preferred tool (Visual C++, WinDbg, etc.), open Python's executable, setting up the arguments to run your <code>.py</code> file, and enable a breakpoint on the first statement of the function before running the application. Then run it and step through each instruction.</p> <p>It's really hard to guess what's different about your native C++ application and Python, but it may be that different COM initialization arguments are used by Python and <code>doSomethingWithCOMobj</code>, or that you haven't declared it <code>__stdcall</code> (although being a void function that shouldn't matter), or that it tries to write to stdout and you're using <code>pythonw.exe</code> which isn't a console application, etc.</p> <hr> <p><sup>1.</sup> <code>HKEY_CLASSES_ROOT</code> is a mix of <code>HKEY_CURRENT_USER\Software\Classes</code> and <code>HKEY_LOCAL_MACHINE\Software\Classes</code>.</p>
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    1. COThank you, the problem is that neither my C++ application, nor my Python application do any COM initialisation. If something happens, it's automatic (and visibly doesn't happen when Python is used). I'm merely using this 3rd-party DLL which uses that COM DLL.
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    2. CO@Bruno, that sounds odd. That may mean either that your C++ is actually calling `CoInitialize[Ex]`/`OleInitialize`, or that MyLib.dll or SomeLib.dll is doing it with `COINIT_APARTMENTTHREADED` (ther default of `CoInitialize` and `OleInitialize`) after your Python app called `CoInitializeEx` with `COINIT_MULTITHREADED`. I think DLLs shouldn't ever call these functions by themselves.
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    3. CO@Bruno, as for the actual error, since it looks like a C++ exception code, I'd look at and debug MyLib.dll. If you have Visual C++, you may debug a DLL by running an app that loads and uses it, such as your Python app.
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