'宣言
Public Class GenericDocument (Of PageType As {New, Page}, PagesCollectionType As {New, PagesCollection(Of PageType)}) Inherits DocumentBase(Of PageType) Implements IDocument(Of PageType)
public class GenericDocument<PageType,PagesCollectionType> : DocumentBase<PageType>, IDocument<PageType> where PageType: new(), Page where PagesCollectionType: new(), PagesCollection<PageType>