Problem: When adding a .dll file in ArcMap, a "Cannot add an object" error is returned
When working with COM custom components in ArcMap, adding .dll files manually to the component library through the ‘Add from File’ button on the Customize dialog box in ArcMap may result in the following error:
"Cannot add an object."
This is a generic error message and may occur under a number of circumstances.
The .dll file was selected instead of the .tlb file.
Solution or Workaround
Choose a .tlb file instead of the .dll file. Add the .tlb (type library) file through the 'Add from File' button on the Customize dialog box in ArcMap.