class Program
    {
        static void Main(string[] args)
        {
            InsertText("Insert text into a cell.xlsx", "Inserted Text");
        }

        // Given a document name and text, 
        // inserts a new work sheet and writes the text to cell "A1" of the new worksheet.

        public static void InsertText(string docName, string text)
        {
            // Open the document for editing.
            using (SpreadsheetDocument spreadSheet = SpreadsheetDocument.Open(docName, true))
            {
                // Get the SharedStringTablePart. If it does not exist, create a new one.
                SharedStringTablePart shareStringPart;
                if (spreadSheet.WorkbookPart.GetPartsOfType<SharedStringTablePart>().Count() > 0)
                {
                    shareStringPart = spreadSheet.WorkbookPart.GetPartsOfType<SharedStringTablePart>().First();
                }
                else
                {
                    shareStringPart = spreadSheet.WorkbookPart.AddNewPart<SharedStringTablePart>();
                }

                // Insert the text into the SharedStringTablePart.
                int index = InsertSharedStringItem(text, shareStringPart);

                // Insert a new worksheet.
                WorksheetPart worksheetPart = InsertWorksheet(spreadSheet.WorkbookPart);

                // Insert cell A1 into the new worksheet.
                Cell cell = InsertCellInWorksheet("A", 1, worksheetPart);

                // Set the value of cell A1.
                cell.CellValue = new CellValue(index.ToString());
                cell.DataType = new EnumValue<CellValues>(CellValues.SharedString);

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

        // Given text and a SharedStringTablePart, creates a SharedStringItem with the specified text 
        // and inserts it into the SharedStringTablePart. If the item already exists, returns its index.
        private static int InsertSharedStringItem(string text, SharedStringTablePart shareStringPart)
        {
            // If the part does not contain a SharedStringTable, create one.
            if (shareStringPart.SharedStringTable == null)
            {
                shareStringPart.SharedStringTable = new SharedStringTable();
            }

            int i = 0;

            // Iterate through all the items in the SharedStringTable. If the text already exists, return its index.
            foreach (SharedStringItem item in shareStringPart.SharedStringTable.Elements<SharedStringItem>())
            {
                if (item.InnerText == text)
                {
                    return i;
                }

                i++;
            }

            // The text does not exist in the part. Create the SharedStringItem and return its index.
            shareStringPart.SharedStringTable.AppendChild(new SharedStringItem(new DocumentFormat.OpenXml.Spreadsheet.Text(text)));
            shareStringPart.SharedStringTable.Save();

            return i;
        }

        // Given a WorkbookPart, inserts a new worksheet.
        private static WorksheetPart InsertWorksheet(WorkbookPart workbookPart)
        {
            // Add a new worksheet part to the workbook.
            WorksheetPart newWorksheetPart = workbookPart.AddNewPart<WorksheetPart>();
            newWorksheetPart.Worksheet = new Worksheet(new SheetData());
            newWorksheetPart.Worksheet.Save();

            Sheets sheets = workbookPart.Workbook.GetFirstChild<Sheets>();
            string relationshipId = workbookPart.GetIdOfPart(newWorksheetPart);

            // Get a unique ID for the new sheet.
            uint sheetId = 1;
            if (sheets.Elements<Sheet>().Count() > 0)
            {
                sheetId = sheets.Elements<Sheet>().Select(s => s.SheetId.Value).Max() + 1;
            }

            string sheetName = "Sheet" + sheetId;

            // Append the new worksheet and associate it with the workbook.
            Sheet sheet = new Sheet() { Id = relationshipId, SheetId = sheetId, Name = sheetName };
            sheets.Append(sheet);
            workbookPart.Workbook.Save();

            return newWorksheetPart;
        }

        // Given a column name, a row index, and a WorksheetPart, inserts a cell into the worksheet. 
        // If the cell already exists, returns it. 
        private static Cell InsertCellInWorksheet(string columnName, uint rowIndex, WorksheetPart worksheetPart)
        {
            Worksheet worksheet = worksheetPart.Worksheet;
            SheetData sheetData = worksheet.GetFirstChild<SheetData>();
            string cellReference = columnName + rowIndex;

            // If the worksheet does not contain a row with the specified row index, insert one.
            Row row;
            if (sheetData.Elements<Row>().Where(r => r.RowIndex == rowIndex).Count() != 0)
            {
                row = sheetData.Elements<Row>().Where(r => r.RowIndex == rowIndex).First();
            }
            else
            {
                row = new Row() { RowIndex = rowIndex };
                sheetData.Append(row);
            }

            // If there is not a cell with the specified column name, insert one.  
            if (row.Elements<Cell>().Where(c => c.CellReference.Value == columnName + rowIndex).Count() > 0)
            {
                return row.Elements<Cell>().Where(c => c.CellReference.Value == cellReference).First();
            }
            else
            {
                // Cells must be in sequential order according to CellReference. Determine where to insert the new cell.
                Cell refCell = null;
                foreach (Cell cell in row.Elements<Cell>())
                {
                    if (string.Compare(cell.CellReference.Value, cellReference, true) > 0)
                    {
                        refCell = cell;
                        break;
                    }
                }

                Cell newCell = new Cell() { CellReference = cellReference };
                row.InsertBefore(newCell, refCell);

                worksheet.Save();
                return newCell;
            }
        }

    }

class Program
    {
        static void Main(string[] args)
        {
            InsertText("Insert text into a cell.xlsx", "Inserted Text");
        }

        private static void InsertText(string docName, string text)
        {
            //Instantiating a Workbook object
            Workbook workbook = new Workbook(docName);

            //Obtaining the reference of the Active worksheet
            Worksheet worksheet = workbook.Worksheets[workbook.Worksheets.ActiveSheetIndex];

            //insert value from cell
            worksheet.Cells["A1"].PutValue(text);

            //Saving the Excel file
            workbook.Save(docName);
        }
    }