Insert a new worksheet
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    OpenXML Excel
 string FilePath = @"..\..\..\..\Sample Files\";
string FileName = FilePath + "Insert a new worksheet.xlsx";
InsertWorksheet(FileName);
}
private static void InsertWorksheet(string docName)
{
// Open the document for editing.
using (SpreadsheetDocument spreadSheet = SpreadsheetDocument.Open(docName, true))
{
    // Add a blank WorksheetPart.
    WorksheetPart newWorksheetPart = spreadSheet.WorkbookPart.AddNewPart<WorksheetPart>();
    newWorksheetPart.Worksheet = new Worksheet(new SheetData());
    Sheets sheets = spreadSheet.WorkbookPart.Workbook.GetFirstChild<Sheets>();
    string relationshipId = spreadSheet.WorkbookPart.GetIdOfPart(newWorksheetPart);
    // Get a unique ID for the new worksheet.
    uint sheetId = 1;
    if (sheets.Elements<Sheet>().Count() > 0)
    {
        sheetId = sheets.Elements<Sheet>().Select(s => s.SheetId.Value).Max() + 1;
    }
    // Give the new worksheet a name.
    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);
}
}
Aspose.Cells
string FilePath = @"..\..\..\..\Sample Files\";
string FileName = FilePath + "Insert a new worksheet.xlsx";
InsertWorksheet(FileName);
private static void InsertWorksheet(string docName)
{
  //Instantiating a Workbook object
  Workbook workbook = new Workbook(docName);
  //Adding a new worksheet to the Excel object
  int SheetIndex = workbook.Worksheets.Add();
  //Saving the Excel file
  workbook.Save(docName);
}