C#

public partial class Form1 : Form
  {

    Microsoft.Office.Interop.Excel.Application _objAppln;
    Workbook _objWorkBook;
    Workbooks _objWorkBooks;
    Worksheet _objWorkSheet;


    public Form1()
    {
      InitializeComponent();
      InitializeExcelObjectModel();
    }

    ~Form1()
    {
      DisposeExcelObjects();
    }

    void SaveExcel()
    {

      _objAppln.DisplayAlerts = false;//Since, we are using SaveFileDialog's overwrite prompt(control is on view). 
      
      _objWorkBook.SaveAs("C:\\tmp.xls",
        XlFileFormat.xlWorkbookNormal, Type.Missing, Type.Missing, false, Type.Missing, XlSaveAsAccessMode.xlNoChange,
        Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
      _objWorkBook.Close(true, "C:\\tmp.xls", false);
      _objAppln.DisplayAlerts = true;//restore back for other display alerts 
    }

    private void InitializeExcelObjectModel()
    {

      _objAppln = new Microsoft.Office.Interop.Excel.Application(); // To initialize excel file   
      //_objAppln.Visible = true; 
      if (_objAppln != null)
      {
        _objWorkBooks = _objAppln.Workbooks;
        _objWorkBook = _objWorkBooks.Add(Type.Missing); // To add workbook with sheets in excel file         
        _objWorkSheet = (Worksheet)_objAppln.ActiveSheet; // To get the current active sheet in excel file              

      }
    }

    public void DisposeExcelObjects()
    {

      System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkSheet);
      System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkBook);
      System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkBooks);
      System.Runtime.InteropServices.Marshal.ReleaseComObject(_objAppln);
      _objWorkSheet = null; _objWorkBooks = null; _objWorkBooks = null; _objAppln = null;
    }

    private void button1_Click(object sender, EventArgs e)
    {

      Range objRange=null;
      string cell1 = string.Empty, cell2 = string.Empty;
      string[] chars = { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T" };


      for (int row = 1; row < 1000; row++)
      {
        for (int column = 0; column < 20; column++)
        {
          cell1 = chars[column] + row.ToString();
          objRange = _objWorkSheet.get_Range(cell1, cell1);
          objRange.Value2 = cell1;
          
          
        }
      }
      System.Runtime.InteropServices.Marshal.ReleaseComObject(objRange);
      objRange = null;
      SaveExcel();
    }
  }