Cannot add custom controls to webform?

A strange situation was found yesterday. The custom control added to the custom control library cannot be added to the webform in any way, but it is only a new control. The old one is OK. This is also true for writing several simplest controls !!

No reason was found after one day. I came back this morning and looked at the DLL file of the referenced control library. I wanted to delete it. The reference of the library file is deleted from the reference, and the control is added to webform. The control is added to the webform as expected, and the DLL is automatically added to the reference.

This is the case. Previously, I did not add a control project to this solution, but copied its DLL file to a DLLs directory in the solution for use. Of course, when a new control is added to the control library, the DLL referenced by the project where the webform is located is not updated. This is the case.