For developers: Reference the template of the simple widget designer | WebForms

To wire up the template with the designer, you must return its name via the LayoutTemplatePath property. It specifies the path to the embedded resource following the naming convention <Namespace>.<ResourceFolderPath>.<TemplateName>, where you must replace path slashes inside the ResourceFolderPath with dots. 

For example, if you have project called SimpleWidget and the path to the template file is Resources/Views/SimpleWidgetDesigner.ascx, the LayoutTemplatePath property must return the following:
SimpleWidget.Resources.Views.SimpleWidgetDesigner.ascx

EXAMPLE: For more information about referencing the template of the designer in the designer's class, using the LayoutTemplatePath property, see For developers: Full code of the widget designer class.