If you are a new user of UIPath and face an issue when you try to create a custom namespace and import it, but failing in the process, try the following steps:
Choose c# Class Library
Add references system.activities and system.componentModel
Write the code that you want to execute
Define the input and output parameters
Build your solution
Go to the holding folder and open bin-folder
Open the NuGet explorer and create new library and add the DLL by choosing the add existing option.
Click to write and change the title of the package
Save the file with the desired title and copy the file(.nupckg extension) to the path C:\Users\><user>\AppData\Local\UiPath\app-18.4.0\Packages
Open UiPath and search for the name.
Then, click install. Now, you can execute the code giving the inputs.