I\'ve been building DLL class libraries in C#, used as add-ons to an application which provides a Custom API. Up until now they\'ve included mostly interfacing with databases,
You can definitely use Windows Forms inside your class library. There are few scenarios:
You are adding a form to the library using FormDesigner. You can right click on project name, click on Add and then in Windows form. It should add required References for your form
You are copying a form from other project. In this case the Visual Studio will not be able to identify the form and will show it as simple C# source file. In this case right click on References under the project in Visual Studio. Click on Add References and select Framework
from left pane. Select System.Windows.Form
and System.Drawing
and click Ok. This should make the form understandable to Visual Studio and you can design it edit it using Form Designer.
System.Windows.Form
in your project with using System.Windows.Form;
statement.