_Application.ThisWorkbook property

Returns a Workbook object that represents the workbook where the current macro code is running.

Namespace:  Microsoft.Office.Interop.Excel
Assembly:  Microsoft.Office.Interop.Excel (in Microsoft.Office.Interop.Excel.dll)

Workbook ThisWorkbook { get; }

Use this property to refer to the workbook that contains your macro code. ThisWorkbook is the only way to refer to an add-in workbook from inside the add-in itself. The ActiveWorkbook property doesn't return the add-in workbook; it returns the workbook that's calling the add-in. The Workbooks property may fail, because the workbook name probably changed when you created the add-in. ThisWorkbook always returns the workbook in which the code is running.

This property can be used only from inside Excel. You cannot use it to access a workbook from any other application.