Namespace 'Excel"could not be found

Hi, error message was
" The type or namespace "Excel" could not be found( are you missing ua using directive or an assembly reference?)"
I did remove the reference "Microsoft/Office.Core" and add back. Initial the program was developed with MS Office 2003. Currently i using the same program in MS Office 2007.  

Still after add back i am facing the same error. Please do advice.
Rama TitoProgrammerAsked:
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Do you have the correct using included in your code?

using Microsoft.Office.Core

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Rama TitoProgrammerAuthor Commented:
After i include " using Microsoft.Office.core" the error still indicate the same.
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Data.OleDb;
using System.IO;
using System.Threading;
using System.Configuration;	
using System.Data.SqlClient;
using System.Globalization;
using System.Diagnostics;
using Microsoft.Office.Core;

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Sorry, I should clarify.

I don't know where the Excel class exists.  Does it exist in Microsoft.Office.Core?  Apparently not.  That being said, are the libraries that are used for Excel interop functionality the same for 2003 and 2007?

It would appear as though the namespace to be used is:

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and I assume there's a Microsoft.Office.Interop reference you need to add.

From there, it may be the case that the Excel.Worksheet you have in your code is the result of declaring the using statement such as:

using Excel = Microsoft.Office.Interop.Excel

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but I can't say for sure

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Miguel OzSenior Software EngineerCommented:
All excel classes like Workbook, Worksheet are defined at Microsoft.Office.Interop.Excel; you have to use:
 using Excel = Microsoft.Office.Interop.Excel
and you need to add the version 12  of this assembly as a reference(Excel 2007).
If your add reference dialog does not have it (as per my screenshot), you need to install Excel 2007PIA from:
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