ExcelApplication.Open

Overload	Description
Open(ExcelTemplate)	Opens a Workbook from an ExcelTemplate object.
Open(String)	Opens an existing Excel spreadsheet.
Open(String, String)	Opens an existing Excel spreadsheet.
Open(System.IO.Stream)	Opens an Excel workbook from a stream and returns a Workbook object.
Open(System.IO.Stream, String)	Opens an Excel workbook from a stream and returns a Workbook object.

Overload	Description
Open(Microsoft.SharePoint.SPDocumentLibrary, String)	Opens an existing Excel spreadsheet from a SharePoint Document Library.
Open(Microsoft.SharePoint.SPListItem, String)	Opens an existing Excel spreadsheet that is attached to a SharePoint List Item.