Error: There are no components in 'MyDll.dll that can be placed on the toolbox.

Issue

You will receive the above error message if there is no accessible class in your DLL and you are trying to add the DLL as a control to your IDE toolbox.

 

Solution

Make at least one of the classes public so it can be accessed from out side of the class.

 

It is a very simple problem but if you do not know what exactly is causing it, it can get really annoying.