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using System;
using System.Windows.Forms;
using System.Reflection;
using Excel = Microsoft.Office.Interop.Excel;
using System.Runtime.InteropServices;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private const string EXCEL_FILE = @"C:\Dokumente und Einstellungen\daniel.gorski\Desktop\Test.xlsx";

        Excel.ApplicationClass exApp;
        Excel.Workbook exWb;
        Excel.Worksheet exWs;

        object missing = Missing.Value;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                initExcelApplication();
                initExcelWorkbook();
                initExcelWorksheet();

                exApp.DisplayAlerts = false;

                exWb = openExcelWorkbook(EXCEL_FILE);

                writeExcelCell(1, 1, "Name:");
                writeExcelCell(1, 2, "Vorname:");

                writeExcelCell(3, 1, "Presley");
                writeExcelCell(3, 2, "Elvis");

                saveExcelWorkbookAs(exWb, EXCEL_FILE);

                closeAllExcelWorkbooks();
            }
            finally
            {
                exApp.Quit();
                freeExcelRam();
            }
        }

        //write to a cell of current workbook
        private void writeExcelCell(int row, int column, string value)
        {
            if (exWs != null)
            {
                exWs.Cells[row, column] = value;
            }
        }

        //removes all Excel-Elements from RAM
        private void freeExcelRam()
        {
            foreach (Excel.Workbook workbook in exApp.Workbooks)
            {
                Marshal.ReleaseComObject(workbook);
            }
            Marshal.FinalReleaseComObject(exApp);
        }

        private void closeAllExcelWorkbooks()
        {
            foreach (Excel.Workbook workbook in exApp.Workbooks)
            {
                workbook.Close(true, EXCEL_FILE, missing);
            }
        }

        private void saveExcelWorkbookAs(Excel.Workbook workbook, string filename)
        {
            workbook.SaveAs(filename, missing, missing, missing, missing, missing, Excel.XlSaveAsAccessMode.xlNoChange, missing, missing, missing, missing, missing);
        }

        private Excel.Workbook openExcelWorkbook(string filename)
        {
            return exApp.Workbooks.Open(filename, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing);
        }

        //initializes an worksheet in the workbook
        private void initExcelWorksheet()
        {
            if (exWb != null)
            {
                exWs = (Excel.Worksheet)exWb.ActiveSheet;
            }
        }

        //initializes an workbook in Excel
        private void initExcelWorkbook()
        {
            if (exApp != null)
            {
                exWb = exApp.Workbooks.Add(Missing.Value);
            }
        }

        //initializes an Excel-Application
        private void initExcelApplication()
        {
            exApp = new Excel.ApplicationClass();
        }
    }
}					

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private void closeAllExcelWorkbooks()
{
    foreach (Excel.Workbook workbook in exApp.Workbooks)
    {
        workbook.Close(false, missing, missing);
    }
}

private void saveExcelWorkbookAs(Excel.Workbook workbook, string filename)
{
    foreach (Excel.Worksheet worksheet in workbook.Worksheets)
    {
        worksheet.SaveAs(EXCEL_FILE, missing, missing, missing, missing, missing, missing, missing, missing, missing);
    }

    workbook.SaveAs(filename, missing, missing, missing, missing, missing, Excel.XlSaveAsAccessMode.xlNoChange, missing, missing, missing, missing, missing);
}