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Gets or sets the external workbook reference.
Use this property to retrieve or update the workbook component of an external reference,
such as a reference that points to a worksheet in another workbook. The default
value is null.
public WorkbookReference Workbook { get; set; }
Public Property Workbook As WorkbookReference
Reference reference = new Reference();
reference.Workbook = WorkbookReference.FromName("Book1");
WorkbookReference workbookReference = reference.Workbook;