Excel – Using Open XML how do you insert a formula into an Excel 2010 worksheet

Here is the working code. This is a prototype. For a larger number of changes, one might open the document only once. Also, there are some hard-coded things like sheet name and cell type that would have to be parameterized before this can be called production-ready. http://openxmldeveloper.org/forums/4005/ShowThread.aspx was very helpful.

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Spreadsheet;
using System.Xml;
using System.IO;
using System.Diagnostics;

namespace OpenXMLWindowsApp
{
    public class OpenXMLWindowsApp
    {
        public void UpdateSheet()
        {
            UpdateCell("Chart.xlsx", "20", 2, "B");
            UpdateCell("Chart.xlsx", "80", 3, "B");
            UpdateCell("Chart.xlsx", "80", 2, "C");
            UpdateCell("Chart.xlsx", "20", 3, "C");

            ProcessStartInfo startInfo = new ProcessStartInfo("Chart.xlsx");
            startInfo.WindowStyle = ProcessWindowStyle.Normal;
            Process.Start(startInfo);
        }

        public static void UpdateCell(string docName, string text,
            uint rowIndex, string columnName)
        {
            // Open the document for editing.
            using (SpreadsheetDocument spreadSheet = 
                     SpreadsheetDocument.Open(docName, true))
            {
                WorksheetPart worksheetPart = 
                      GetWorksheetPartByName(spreadSheet, "Sheet1");

                if (worksheetPart != null)
                {
                    Cell cell = GetCell(worksheetPart.Worksheet, 
                                             columnName, rowIndex);

                    cell.CellValue = new CellValue(text);
                    cell.DataType = 
                        new EnumValue<CellValues>(CellValues.Number);

                    // Save the worksheet.
                    worksheetPart.Worksheet.Save();
                }
            }

        }

        private static WorksheetPart 
             GetWorksheetPartByName(SpreadsheetDocument document, 
             string sheetName)
        {
            IEnumerable<Sheet> sheets =
               document.WorkbookPart.Workbook.GetFirstChild<Sheets>().
               Elements<Sheet>().Where(s => s.Name == sheetName);

            if (sheets.Count() == 0)
            {
                // The specified worksheet does not exist.

                return null;
            }

            string relationshipId = sheets.First().Id.Value;
            WorksheetPart worksheetPart = (WorksheetPart)
                 document.WorkbookPart.GetPartById(relationshipId);
            return worksheetPart;

        }

        // Given a worksheet, a column name, and a row index, 
        // gets the cell at the specified column and 
        private static Cell GetCell(Worksheet worksheet, 
                  string columnName, uint rowIndex)
        {
            Row row = GetRow(worksheet, rowIndex);

            if (row == null)
                return null;

            return row.Elements<Cell>().Where(c => string.Compare
                   (c.CellReference.Value, columnName + 
                   rowIndex, true) == 0).First();
        }


        // Given a worksheet and a row index, return the row.
        private static Row GetRow(Worksheet worksheet, uint rowIndex)
        {
            return worksheet.GetFirstChild<SheetData>().
              Elements<Row>().Where(r => r.RowIndex == rowIndex).First();
        } 
    }
}

I used the code provided by Andrew J, but the DataType CellValues.Date produced a corrupted xlsx-file for me.

The DataType CellValues.Number worked fine for me (Don't forget to set NumberFormatId):

cell.DataType = new EnumValue<CellValues>(CellValues.Number);

My whole code:

DateTime valueDate = DateTime.Now;
string valueString = valueDate.ToOADate().ToString();
CellValue cellValue = new CellValue(valueString);

Cell cell = new Cell();
cell.DataType = new EnumValue<CellValues>(CellValues.Number);
cell.StyleIndex = yourStyle; //StyleIndex of CellFormat cfBaseDate -> See below
cell.Append(cellValue);

My CellFormat for this cell in the Stylesheet looks like:

CellFormat cfBaseDate = new CellFormat() { 
 ApplyNumberFormat = true,
 NumberFormatId = 14, //14 is a localized short Date (d/m/yyyy) -> See list below
 //Some further styling parameters
}; 

If you'd like to format your date another way, here is a list of all default Excel NumberFormatId's

ID  FORMAT CODE
0   General
1   0
2   0.00
3   #,##0
4   #,##0.00
9   0%
10  0.00%
11  0.00E+00
12  # ?/?
13  # ??/??
14  d/m/yyyy
15  d-mmm-yy
16  d-mmm
17  mmm-yy
18  h:mm tt
19  h:mm:ss tt
20  H:mm
21  H:mm:ss
22  m/d/yyyy H:mm
37  #,##0 ;(#,##0)
38  #,##0 ;[Red](#,##0)
39  #,##0.00;(#,##0.00)
40  #,##0.00;[Red](#,##0.00)
45  mm:ss
46  [h]:mm:ss
47  mmss.0
48  ##0.0E+0
49  @

Source of list: https://github.com/ClosedXML/ClosedXML/wiki/NumberFormatId-Lookup-Table

I know this list is from ClosedXML, but it's the same in OpenXML.