Why? Because Inventor already has the correct version in its own installation directory. Your add-in should always use the interop DLL . If you copy it locally, your add-in might load a mismatched version, leading to System.InvalidCastException or MissingMethodException at runtime.
using Inventor; using System.Runtime.InteropServices; autodesk.inventor.interop.dll
: If your app fails on other machines, check that "Embed Interop Types" is set correctly in your project settings—setting it to "False" while setting "Copy Local" to "True" is often required for complex automation tasks [5.19]. autodesk.inventor.interop.dll