How to Convert XLS to XLSX in C#. For more information, please follow link: https://kb.aspose.com/cells/net/how-to-convert-xls-to-xlsx-in-csharp/

How to Convert XLS to XLSX in C#.cs

using Aspose.Cells;

namespace ConvertXlsToXlsInCSharp
{
    class Program
    {
        static void Main(string[] args)
        {
            // Instantiate the license to avoid trial version watermark in the output XLSX
            License cellsLicense = new License();
            cellsLicense.SetLicense("Aspose.Cells.lic");

            // Create LoadOptions class object and initialize with LoadFormat for XLS 
            LoadOptions options = new LoadOptions(LoadFormat.Excel97To2003);

            // Set memory settings to MemoryPreference
            options.MemorySetting = MemorySetting.MemoryPreference;
            
            // Set the flag to parse formula when source XLS file is opened
            options.ParsingFormulaOnOpen = true;

            // Set the password to read the source XLS file
            options.Password = "1234";

            // Set filter to load the sheets with picture only
            options.LoadFilter.LoadDataFilterOptions = LoadDataFilterOptions.Picture;

            // Open the source XLS file with the desired filters
            Workbook workbook = new Workbook("PasswordProtectAndCharts.xls",options);

            // Save the resultant workbook as XLSX
            workbook.Save("XlsxOutput.xlsx");

            System.Console.WriteLine("Done");
        }    
    }
}