Coming from NPOI

NPOI has three families that matter here: XSSF* (OOXML workbooks), HSSF* (legacy .xls) and XWPF* (Word documents). OneDocument has one model per kind (Workbook, Document, Presentation) that opens whichever file format holds it. Read README.md first for what is true on every page.

Workbooks: open, read, write, save

// NPOI
using var fs = File.OpenRead(path);
IWorkbook wb = new XSSFWorkbook(fs);          // HSSFWorkbook for .xls, or WorkbookFactory.Create
ISheet sheet = wb.GetSheet("Sales");
IRow row = sheet.GetRow(1) ?? sheet.CreateRow(1);
ICell cell = row.GetCell(1) ?? row.CreateCell(1);
cell.SetCellValue(1200);
using var outFs = File.Create(outPath);
wb.Write(outFs);

// OneDocument
using Workbook wb = Workbook.Open(path);      // .xlsx, .xlsb and .xls all open as a Workbook
Worksheet ws = wb.Sheets["Sales"];
ws[2, 2].Value = 1200;                        // NPOI rows and columns are 0-based; here they are 1-based
wb.SaveAs(outPath);                           // .xlsx, .xlsb, .xls, .csv or .pdf by extension

No row or cell creation step: assigning a value creates the cell. Row and column numbers are 1-based.

Formulas and evaluation

// NPOI
cell.SetCellFormula("SUM(B2:B4)");
wb.GetCreationHelper().CreateFormulaEvaluator().EvaluateAll();

// OneDocument
ws["B5"].Formula = "SUM(B2:B4)";
wb.RecalculateAll();

Styles

// NPOI
ICellStyle style = wb.CreateCellStyle();
style.DataFormat = wb.CreateDataFormat().GetFormat("#,##0.00");
cell.CellStyle = style;

// OneDocument
ws["B2"].UpdateStyle(s => s with { NumberFormatCode = "#,##0.00" });

The workbook's style table is managed for you; a style is a record, not a shared object to create and attach.

Legacy .xls

OneDocument reads BIFF2 to BIFF8 and writes one BIFF8 dialect (wb.SaveAs("out.xls"), or with a password for RC4 encryption). HSSF's low-level record classes have no counterpart; the record layer is RecursiveAI.OneDocument.Spreadsheets.Biff (docs/xls.md).

Word documents (XWPF)

// NPOI
using var fs = File.OpenRead(path);
var doc = new XWPFDocument(fs);
foreach (XWPFParagraph p in doc.Paragraphs) { Console.WriteLine(p.Text); }
XWPFParagraph np = doc.CreateParagraph();
np.CreateRun().SetText("Added");
using var outFs = File.Create(outPath);
doc.Write(outFs);

// OneDocument
using Document doc = Document.Open(path);       // also .doc, .rtf, .odt
foreach (Paragraph p in doc.Paragraphs) { Console.WriteLine(p.Text); }
doc.Body.AddParagraph("Added");
doc.SaveAs(outPath);

Paragraph style is p.Style (the name Word shows, "Heading 1") or p.StyleId; Body.AddParagraph(text) adds a paragraph. doc.Body.AddTable(rows) creates a table and doc.Tables edits cells and rows (docs/docx-tables-sections.md). A run is formatted with run.Bold, run.Italic, run.Underline, run.FontSize, run.Color and run.FontName; the typed view run.View reaches everything else (docs/authoring.md).

Text extraction

// NPOI (the extractor classes)
var extractor = new XWPFWordExtractor(doc);
string text = extractor.Text;

// OneDocument
string text = Office.ExtractText(path);          // any supported format, streaming where it can

Streaming

NPOI's SXSSFWorkbook streams writes. OneDocument's XlsxStreamWriter and XlsxStreamReader stream both ways (docs/spreadsheets-streaming.md).

Not supported or different

• No HSLF/XSLF (PowerPoint) object model matching: use Presentation (docs/presentations.md); .ppt is read-only.
• No writing of .doc or .ppt.
• Pictures are added with body.AddPicture(bytes, widthPoints, ...), sheet.AddPicture(bytes, "E1") and slide.AddPicture(bytes, left, top, width); there is no drawing-shape or chart creation call (existing ones are kept and rendered).
• No IRow / IFormulaEvaluator objects; no ICellStyle instances.
• .xls support is BIFF8 for writing; the formula evaluation engine is not NPOI's, and results are documented in docs/spreadsheets-calc.md.