When to set AspCompat

If we interop with a COM component from ASP.NET, we might need to use AspCompat="true" in the @ Page directive. The question is: when do we need AspCompat?

The answer is that we need AspCompat if the COM component has to run on a thread initialized into a single threaded apartment (STA). This begs the question: how do we know if a component needs to run in an STA?

One answer is to dig into the HKEY_CLASSES_ROOT section of the registry and find the component by CLSID (or by ProgID, which can give us the CLSID). Inside will be an InprocServer32 key, which holds a ThreadingModel value.

We could also find this value using the OLE/COM Object Viewer. Locate the component on the left, and look for the ThreadingModel setting on the right.

If the ThreadingModel value is "Apartment", we need to use AspCompat="true" (and look for ways to get rid of the component). If the value is "Free", "Neutral", or "Both", then we don't need AspCompat.

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