CastError when calling interop Excel functions in a thread?-Collection of common programming errors

When you use an Excel interface method from a worker thread then the method call needs to be marshaled. In other words, the call needs to be made from the thread that created the Application object. Same idea as using Control.Invoke() or Dispatcher.Invoke() in a .NET gui app.

To make that work, COM needs to know what the arguments are for the method so it can copy them properly. That is the kind of info that requires the equivalent of Reflection in .NET. Which requires metadata, the metadata that describes a COM method is stored in a type library. The type library for Excel is stored in Excel.exe as a resource.

Finding the type library is what is going wrong here. This info is stored in the registry and it is damaged on your machine for some reason. The most likely key that got whacked is HKLM\SOFTWARE\Classes\TypeLib\{00020813-0000-0000-C000-000000000046}\1.7\0\win32 albeit that it depends on the exact version of Office. You can get more insight from SysInternals’ ProcMon utility, you’ll see your program searching for the key.

This kind of mishap is rarely limited to just one key. You’ll need to get your machine healthy again and reinstall Office.

Oh, and keep in mind that the code doesn’t actually run on the worker thread. That requires creating the Application object on that worker thread and calling SetApartmentState() to make it STA before you start it.