My user is trying to register custom toolbar assembly developed in 10.5.1 on ArcMap 10.6 using ESRIRegASM but uable to do so, as it is giving error message, "Registration failed. Could not load the file or assembly myfilename.dll or one of its dependencies. Operation is not supported. (Excpetion from HRESULT: 0X80131515)"
My assumption is that it is possible to register lower version assembly in higher version of ArcMap.
Can anybody help with this or any solution.