Export the file in XLS format or convert the xls file to xlsx format in c#.net

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C#

ASP.NET

I have a code that generate xls file using XML formatting (with styles and coloring in celles). Now with out changing the entire code I need to export the excel to xlsx format. I cant use interop. I can use third party dll but expecting any solution without using the third party dll.

Current code that generate xls file is given below,

StringBuilder xmlStrBuilder = new StringBuilder();
xmlStrBuilder.Append("");
xmlStrBuilder.Append("<?mso-application progid=\"Excel.Sheet\"????>");
xmlStrBuilder.Append("<workbook xmlns="\" hold=" />            xmlStrBuilder.Append(" xmlns:o="\" xmlns:x="\"urn:schemas-microsoft-com:office:excel\"");<br" mode="hold" />            xmlStrBuilder.Append(" xmlns:ss=\"urn:schemas-microsoft-com:office:spreadsheet\"");
xmlStrBuilder.Append(" xmlns:html=\"http://www.w3.org/TR/REC-html40\">");
-------------
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xmlStrBuilder.Append("<worksheet>");

response.Clear();
response.AppendHeader("Content-Type", "application/vnd.ms-excel");
response.AppendHeader("Content-disposition", "attachment; filename=" + fileName);
response.Write(xmlStrBuilder);
response.Flush();
response.End();
excelXml.Length = 0;
excelXml.Capacity = 0;

What I have tried:

I tried to use the third party dll which is not converting the file as expected. Changing the mime type to openxmlformat is not working. Any option to use officeopenxml to convert the file.