So, i’m using NPOI to generate an XLSX file, but i note that it’s very slow with so mutch data. I’ve created a workaround using CSV, but it’s not good.

I learning about XLSX, and i can ‘understand struct’ xlsx file, is many XML to build a XLSX.

I try build xlsx on thats way:

public static FileResult DownloadXlsxFile_StringBuilder(this ControllerBase controller, object data,
        string fileName = "file.xlsx")
    {
        // um arquivo XLSX nada mais é que um XLM compactado e zipado.

        if (data is IEnumerable<object> dataList)
        {
            var sheetXml = new StringBuilder();
            sheetXml.Append("""
                            <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
                                        <worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
                                            <sheetData>
                            """);

            // draw header
            PropertyInfo[] props = dataList.First().GetType().GetProperties();
            sheetXml.Append("<row>");
            for (int i = 0; i < props.Length; i++)
            {
                sheetXml.Append($"""<c r="{(char)('A' + i)}1" t="inlineStr"><is><t>{props[i].Name}</t></is></c>""");
            }

            sheetXml.Append("</row>");

            // draw rows
            int indexRow = 1;
            foreach (var dataItem in dataList)
            {
                IEnumerable<object> values =
                    props.Select(x => dataItem.GetType().GetProperty(x.Name)?.GetValue(dataItem, null));
                sheetXml.Append("<row>");
                int indexCol = 0;
                foreach (var value in values)
                {
                    sheetXml.Append(
                        $"""<c r="{(char)('A' + indexCol)}{indexRow + 1}" t="inlineStr"><is><t>{value}</t></is></c>""");
                    indexCol++;
                }

                sheetXml.Append("</row>");
                indexRow++;
            }

            sheetXml.Append("</sheetData></worksheet>");

            Console.WriteLine(sheetXml.ToString());

            // draw content to Xlsx
            using (var memoryStream = new MemoryStream())
            {
                using (var archive = new ZipArchive(memoryStream, ZipArchiveMode.Create, true))
                {
                    // Add xl/worksheets/sheet1.xml
                    var sheetXmlEntry = archive.CreateEntry("xl/worksheets/sheet1.xml");
                    using (var stream = sheetXmlEntry.Open())
                    using (var writer = new StreamWriter(stream, Encoding.UTF8))
                    {
                        writer.Write(sheetXml.ToString());
                    }

                    // Add xl/_rels/workbook.xml.rels
                    var workbookRelsXml = archive.CreateEntry("xl/_rels/workbook.xml.rels");
                    using (var stream = workbookRelsXml.Open())
                    using (var writer = new StreamWriter(stream, Encoding.UTF8))
                    {
                        writer.Write(@"<?xml version=""1.0"" encoding=""UTF-8"" standalone=""yes""?>
                        <Relationships xmlns=""http://schemas.openxmlformats.org/package/2006/relationships"">
                            <Relationship Id=""rId1"" Type=""http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet"" Target=""worksheets/sheet1.xml""/>
                        </Relationships>");
                    }

                    // Add xl/workbook.xml
                    var workbookXml = archive.CreateEntry("xl/workbook.xml");
                    using (var stream = workbookXml.Open())
                    using (var writer = new StreamWriter(stream, Encoding.UTF8))
                    {
                        writer.Write(@"<?xml version=""1.0"" encoding=""UTF-8"" standalone=""yes""?>
                        <workbook xmlns=""http://schemas.openxmlformats.org/spreadsheetml/2006/main"">
                            <sheets>
                                <sheet name=""Sheet1"" sheetId=""1"" r:id=""rId1""/>
                            </sheets>
                        </workbook>");
                    }

                    // Add _rels/.rels
                    var relsXml = archive.CreateEntry("_rels/.rels");
                    using (var stream = relsXml.Open())
                    using (var writer = new StreamWriter(stream, Encoding.UTF8))
                    {
                        writer.Write(@"<?xml version=""1.0"" encoding=""UTF-8"" standalone=""yes""?>
                        <Relationships xmlns=""http://schemas.openxmlformats.org/package/2006/relationships"">
                            <Relationship Id=""rId1"" Type=""http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument"" Target=""xl/workbook.xml""/>
                        </Relationships>");
                    }

                    // Add [Content_Types].xml
                    var contentTypesXml = archive.CreateEntry("[Content_Types].xml");
                    using (var stream = contentTypesXml.Open())
                    using (var writer = new StreamWriter(stream, Encoding.UTF8))
                    {
                        writer.Write(@"<?xml version=""1.0"" encoding=""UTF-8"" standalone=""yes""?>
                        <Types xmlns=""http://schemas.openxmlformats.org/package/2006/content-types"">
                            <Default Extension=""rels"" ContentType=""application/vnd.openxmlformats-package.relationships+xml""/>
                            <Default Extension=""xml"" ContentType=""application/xml""/>
                            <Override PartName=""/xl/workbook.xml"" ContentType=""application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml""/>
                            <Override PartName=""/xl/worksheets/sheet1.xml"" ContentType=""application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml""/>
                        </Types>");
                    }
                }

                memoryStream.Position = 0;
                return controller.File(memoryStream.ToArray(),
                    "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", fileName);
            }
        }

        throw new Exception("cannot convert data to IEnumerable<T> in DownloadXlsxFile");
    }

When i try to open .xlsx file with LibreOfficeCalc, i have: General Error, General Error Input/Output