How to convert Multiple Xlsx file to Xls

Hello,

You can use Spire.XLS package to easily convert xlsx to xls and vice versa.

Install-Package Spire.Xls

Here is an example that converts an xlsx file to xls format:

using Spire.Xls;

namespace ConvertXlsxToXls
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a Workbook instance
            Workbook workbook = new Workbook();
            //Load an XLSX file
            workbook.LoadFromFile("Input.xlsx");

            //Convert the file to XLS format
            workbook.SaveToFile("ToXls.xls", ExcelVersion.Version97to2003);
        }
    }
}

More details can be found in this documentation: 

https://www.e-iceblue.com/Tutorials/Spire.XLS/Spire.XLS-Program-Guide/Excel-Conversion/NET-Excel-Conversion-between-Excel-97-2003.xls-and-Excel-2007.xlsx.html

Hello Feroz,

Please use this code, use EPPlus Nuget package

using System;
using System.IO;
using OfficeOpenXml;
using OfficeOpenXml.ExcelPackage;

class Program
{
    static void Main()
    {
        // XLSX files folder path
        string inputDirectory = @"C:\Path\To\Your\XLSX\Files";

        // output path for for XLS files
        string outputDirectory = @"C:\Path\To\Your\Output\Directory";

        // Ensure the output directory exists
        Directory.CreateDirectory(outputDirectory);

        // Read all XLSX files in the input directory
        string[] xlsxFiles = Directory.GetFiles(inputDirectory, "*.xlsx");

        foreach (var xlsxFile in xlsxFiles)
        {
           
            FileInfo xlsxFileInfo = new FileInfo(xlsxFile);

            // Create the corresponding output XLS file path
            string xlsFilePath = Path.Combine(outputDirectory, Path.ChangeExtension(xlsxFileInfo.Name, "xls"));

            // Convert XLSX to XLS file
            ConvertXlsxToXls(xlsxFileInfo.FullName, xlsFilePath);
        }

        Console.WriteLine("Conversion complete.");
    }

    static void ConvertXlsxToXls(string xlsxFilePath, string xlsFilePath)
    {
        // Load the XLSX file using EPPlus
        using (var package = new ExcelPackage(new FileInfo(xlsxFilePath)))
        {
            // Save the workbook as XLS format
            using (var xlsFile = new FileStream(xlsFilePath, FileMode.Create))
            {
                package.SaveAs(xlsFile);
            }
        }
    }
}

I would also recommend NPOI as a quite useful package 

Microsoft Office Interop does'nt work on azure appservices

First, you'll need to install the required NuGet packages. In your Visual Studio project, open the Package Manager Console and run the following commands:

Install-Package EPPlus
Install-Package NPOI

Once the packages are installed, you can use the following code to convert a .xlsx file to .xls:

using System;
using System.IO;
using OfficeOpenXml;
using NPOI.HSSF.UserModel;
using NPOI.SS.UserModel;

class Program
{
    static void Main()
    {
        string inputFilePath = "path/to/input/file.xlsx";
        string outputFilePath = "path/to/output/file.xls";

        ConvertXlsxToXls(inputFilePath, outputFilePath);

        Console.WriteLine("Conversion complete.");
    }

    static void ConvertXlsxToXls(string inputFilePath, string outputFilePath)
    {
        using (var excelPackage = new ExcelPackage(new FileInfo(inputFilePath)))
        {
            var workbook = excelPackage.Workbook;
            var xlsWorkbook = new HSSFWorkbook();

            foreach (var sheet in workbook.Worksheets)
            {
                var xlsSheet = xlsWorkbook.CreateSheet(sheet.Name);

                for (int row = 1; row <= sheet.Dimension.Rows; row++)
                {
                    var xlsRow = xlsSheet.CreateRow(row - 1);

                    for (int col = 1; col <= sheet.Dimension.Columns; col++)
                    {
                        var cellValue = sheet.Cells[row, col].Text;
                        var xlsCell = xlsRow.CreateCell(col - 1);
                        xlsCell.SetCellValue(cellValue);
                    }
                }
            }

            using (var fs = new FileStream(outputFilePath, FileMode.Create, FileAccess.Write))
            {
                xlsWorkbook.Write(fs);
            }
        }
    }
}

This code uses the EPPlus library to read the .xlsx file and the NPOI library to create a new .xls file with the same data.