Import data with Formula to existing Excel in ASP.Net

Hi,

I am having an existing excel file that needs to be insert or update cell formula.

The excel contains 7 Tab/Spreadsheet only in sheet1 I need to insert formula '=SUM(C1:C5)' The data is exists from A1 to C5.

I need to insert formula in C6. I have declared the C column as Decimal. The formula needs to be inserted in C6 so that when user changes the total it will dynamically update in C6.

I am using OpenXml.

Please help me with this problem. Thanks.

string file = @"E:\Test.xlsx"; 
byte[] bytes = File.ReadAllBytes(file); 
MemoryStream ms = new MemoryStream(); 
ms.Write(bytes, 0, bytes.Length); 
Attachment excelFile = MailManager.CreateAttachment(fileName, MediaTypeNames.Application.Octet, bData); 
attachments.Add(excelFile); 
using (SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.Open(ms, true)) 
{ 
    WorkbookPart workBookPart = spreadsheetDocument.WorkbookPart; 
    WorkbookPart workbookPart = spreadsheetDocument.WorkbookPart; 
    WorksheetPart worksheetPart = workbookPart.WorksheetParts.First(); 
    SheetData sheetData = worksheetPart.Worksheet.Elements<SheetData>().First(); 
    //spreadsheetDocument.WorkbookPart.Workbook.Save(); 
    //find the first row in the sheet data 
    Row row1 = sheetData.GetFirstChild<Row>(); 
 
    //create a new cell 
    Cell cell = new Cell() { CellReference = "D18" }; 
    CellFormula cellformula = new CellFormula(); 
    cellformula.Text = "=SUM(D5:D15)"; 
    cell.Append(cellformula); 
 
    //append the cell to the row 
    row1.Append(cell); 
   workbookPart.Workbook.Save(); 
    spreadsheetDocument.Close(); 
    File.WriteAllBytes(file, ms.ToArray()); 
}