using (SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.Open(spreadSheetFileName, true)) {
WorkbookPart workbookPart = spreadsheetDocument.WorkbookPart;
// rId must be unique within the spreadsheet.
// You might be able to use the SpreadSheetDocument.Parts.Count() to do this.
// i.e. string relationshipID = "rId" + (spreadsheetDocument.Parts.Count() + 1).ToString();
string rId = "rId6";
// Sheet.Name and Sheet.SheetId must be unique within the spreadsheet.
Sheet sheet = new Sheet() { Name = "Sheet4", SheetId = 4U, Id = rId };
workbookPart.Workbook.Sheets.Append(sheet);
WorksheetPart worksheetPart = workbookPart.AddNewPart<WorksheetPart>(rId);
Worksheet worksheet = new Worksheet();
worksheet.AddNamespaceDeclaration("r", "http://schemas.openxmlformats.org/officeDocument/2006/relationships");
// I don't know what SheetDimension.Reference is used for, it doesn't seem to change the resulting xml.
SheetDimension sheetDimension = new SheetDimension() { Reference = "A1:A3" };
SheetViews sheetViews = new SheetViews();
// If more than one SheetView.TabSelected is set to true, it looks like Excel just picks the first one.
SheetView sheetView = new SheetView() { TabSelected = false, WorkbookViewId = 0U };
// I don't know what Selection.ActiveCell is used for, it doesn't seem to change the resulting xml.
Selection selection = new Selection() { ActiveCell = "A1", SequenceOfReferences = new ListValue<StringValue>() { InnerText = "A1" } };
sheetView.Append(selection);
sheetViews.Append(sheetView);
SheetFormatProperties sheetFormatProperties = new SheetFormatProperties() { DefaultRowHeight = 15D };
SheetData sheetData = new SheetData();
// I don't know what the InnerText of Row.Spans is used for. It doesn't seem to change the resulting xml.
Row row = new Row() { RowIndex = 1U, Spans = new ListValue<StringValue>() { InnerText = "1:3" } };
Cell cell1 = new Cell() { CellReference = "A1", DataType = CellValues.Number, CellValue = new CellValue("99") };
Cell cell2 = new Cell() { CellReference = "B1", DataType = CellValues.Number, CellValue = new CellValue("55") };
Cell cell3 = new Cell() { CellReference = "C1", DataType = CellValues.Number, CellValue = new CellValue("33") };
row.Append(cell1);
row.Append(cell2);
row.Append(cell3);
sheetData.Append(row);
PageMargins pageMargins = new PageMargins() { Left = 0.7D, Right = 0.7D, Top = 0.7D, Bottom = 0.75D, Header = 0.3D, Footer = 0.3D };
worksheet.Append(sheetDimension);
worksheet.Append(sheetViews);
worksheet.Append(sheetFormatProperties);
worksheet.Append(sheetData);
worksheet.Append(pageMargins);
worksheetPart.Worksheet = worksheet;
}