Thank you for your reply! However, GetWebResourceUrl: This method is new in the .NET Framework version 2.0. Now I tell you the detail: I want to write a datetimepicker, it has a textbox and an image at beside. So that I create a web control extended textbox. I don't care about the image of the assembly, I just pay attention to the image beside the textbox. I want when I compile it into dll file, It will be a dependent control. The image beside the textbox doesn't belong to any projects. any suggestion?