create xlsx file without third party or office using c#

Creating an XLSX file in C# without using any third-party libraries or Microsoft Office Interop is possible by working with the Open XML format, which is the native format for Excel files since Excel 2007. This can be done by generating the necessary XML structure manually and then saving it as an .xlsx file. Here's a basic example of how you can do this:

Steps to Create XLSX Manually in C#

Create the Required XML Parts:
- You need to create several XML files that represent different parts of the XLSX file, such as the workbook, worksheet, and data.

Package the Parts Together:

The XLSX file is essentially a zip archive that contains multiple XML files and folders. You’ll need to package these XML files into a .zip file and then rename it to .xlsx.

using System;
using System.IO;
using System.IO.Compression;
using System.Text;

class Program
{
  static void Main()
  {
    // Create the necessary folders for the XLSX file structure
    string folderPath = "ExcelFile";
    Directory.CreateDirectory(folderPath);
    Directory.CreateDirectory(Path.Combine(folderPath, "_rels"));
    Directory.CreateDirectory(Path.Combine(folderPath, "xl"));
    Directory.CreateDirectory(Path.Combine(folderPath, "xl", "worksheets"));

    // Create [Content_Types].xml
    string contentTypesXml = @"

  
  
  
";

    File.WriteAllText(Path.Combine(folderPath, "[Content_Types].xml"), contentTypesXml);

    // Create .rels file
    string relsXml = @"

  
";

    File.WriteAllText(Path.Combine(folderPath, "_rels", ".rels"), relsXml);

    // Create workbook.xml
    string workbookXml = @"

  
    
  
";

    File.WriteAllText(Path.Combine(folderPath, "xl", "workbook.xml"), workbookXml);

    // Create sheet1.xml
    string sheet1Xml = @"

  
    
      Hello
      World
    
  
";

    File.WriteAllText(Path.Combine(folderPath, "xl", "worksheets", "sheet1.xml"), sheet1Xml);

    // Create the zip file (XLSX file)
    string xlsxPath = "Output.xlsx";
    if (File.Exists(xlsxPath))
    {
      File.Delete(xlsxPath);
    }

    ZipFile.CreateFromDirectory(folderPath, xlsxPath);

    // Clean up temporary files
    Directory.Delete(folderPath, true);

    Console.WriteLine("XLSX file created successfully.");
  }
}

Explanation:

XML Files: Each XML file represents a different part of the Excel document. [Content_Types].xml specifies the content types. .rels file manages relationships. workbook.xml defines the workbook structure, and sheet1.xml holds the data.
Zip Packaging: After creating these XML files, they are zipped together to form an .xlsx file.

Limitations:

This method works for simple Excel files. If you need more advanced features (like formatting, multiple sheets, etc.), it is better to use libraries like OpenXML SDK or ClosedXML.
This approach allows you to create a basic Excel file programmatically in C# without relying on third-party libraries or Office Interop.