Using dll's as addon's

I was just wondering if it was possible to make a application that can load all dll's located
within the application folder and use them to add controls then use the dll's code for the
control. For example lets say I have a dll named Play_Picturebox.dll when the application loads
the dll it creates a picturebox control named play. Then it uses the dll code to tell the program
what to do when the user clicks play. If this will not work does anyone have an idea off how to
do an application which allows the user to make addon's for it.