My Excel addin (XLL AddIn, call it MyAddIn) is built with C#, ExcelDNA, NetOffice, VS2010. Client has another addin (let's call it B), I guess it is written in VBA. Client says B works fine without MyAddIn. Once MyAddIn is installed, B fails with error:

Error code: 406 Error message: Non-modal forms cannot be displayed in this host application from an ActiveX DLL, ActiveX Control, or Property Page.

I did see this Microsoft resource, but I do not want to just tell the client that B addin needs changing. I want to do something to avoid this from my side.

Here is the steps reported to see the issue:

  1. When B addin is installed, it does not make any registry entry for the Microsoft Excel.
  2. When MyAddin is installed, it makes a registry entry for Microsoft Excel.
  3. Registry entries here basically tells that the addin should be opened when Excel is launched, so B addin is not launched, Excel works fine, MyAddIn works fine.
  4. Now when B addin is launched, it gives the 406 error shown above.
  5. We can ignore the error and keep working with the B addin; disabling MyAddIn is the workaround.
  6. When the B addin is launched, we see that MyAddIn is loaded first before the B addin and then get the 406 error.
  7. When we uninstall MyAddIn, this error is no longer encountered and everything works fine.
  8. To remove this error, we tried changing the registry order to make the B addin always open before MyAddin.
    • This works, but then this a global change for Microsoft
      c# excel excel-addins activex-exe vba
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