Gets a custom extension for this Workbook object.
Assembly: Microsoft.Office.Tools.Excel (in Microsoft.Office.Tools.Excel.dll)
A custom extension is an implementation of the IWorkbookExtension interface in an Excel solution that is created by using the Office development tools in Visual Studio. In document-level projects, this property returns an instance of the generated ThisWorkbook class by default. The ThisWorkbook class implements the IWorkbookExtension interface through the WorkbookBase class. For more information, see Programming Document-Level Customizations.