I have created Custom Activity (as it is mentioned in here https://activities.uipath.com/docs/creating-a-custom-activity). All good when I did sample program.
But when I started creating Custom Activity with many DLLs involved, It kept throwing an error as shown in the attached image. I could able to successfully add that activity in uipath.
The same project is working fine in Visual Studio. This is .Net Class Library and third party DLLs.