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How to convert xls file to xlsx file using C#?

I was developing an application which read data from an excel file, but when I try to open it, an exception  was thrown if the source file is saved with the xls format (File contains corrupted data error when opening Excel sheet with OpenXML). indeed when I save this file with the xlsx format it works fine. please help me to solve this problem.

Use Free Spire.XLS dll available via NuGet.
Sample:
Workbook workbook = new Workbook();
workbook.LoadFromFile("Input.xls");
workbook.SaveToFile("Output.xlsx", ExcelVersion.Version2013);

For reliably reading XLS files you could use ExcelDataReader which is a lightweight and fast library written in C# for reading Microsoft Excel files. It supports the import of Excel files all the way back to version 2.0 of Excel (released in 1987!)
Alternatively you could use a file conversion API like Zamzar. This service has been around for 10+ years, and provides a simple REST API for file conversion - it supports XLS to XLSX conversion. You can use it in C# and it has extra features like allowing you to import and export files to and from Amazon S3, FTP servers etc.
Full disclosure: I'm the lead developer for the Zamzar API.

You cannot read xls files with OpenXML.
The solution from Microsoft is to read the xls file with Office Interop (but Interop is not recommended to be used on the server), transfer data from Interop step by step to OpenXML.
Another solution is to use an Excel library like EasyXLS and convert between these two Excel file formats:
ExcelDocument workbook = new ExcelDocument();
workbook.easy_LoadXLSFile("Excel.xls");
workbook.easy_WriteXLSXFile("Excel.xlsx");
Find more information about converting xls to xlsx.
I am not quite sure why you need to convert the file and why you don't just read the xls file, using a different technology then OpenXML, for sure.

XLS is the older Excel file format. XSLX is the newer format stored as OpenXML. XSLX is actually a zip file with the various components stored as files within it. You cannot simply rename the file to get it into the new format. To save the file in XSLX you'll have to save the file into the Excel 2010+ format.
If you're using Excel interop then it is an option on the SaveAs method.
for more info check the function: _Workbook.SaveAs Method
and the property: FileFormat:
Optional Object.
The file format to use when you save the file. For a list of valid choices,
see the FileFormat property. For an existing file, the default format is the
last file format specified; for a new file, the default is the format of the
version of Excel being used.
msdn info here:
https://msdn.microsoft.com/en-us/library/microsoft.office.interop.excel._workbook.saveas(v=office.11).aspx

How to do Importing and exporting excel using C# and WPF?

I have an Excel file which is generated by a machine. I need to create an application using WPF and C# which can import the excel to see its contents, do some calculations and create some new columns and save back as new excel file. Which is the way to go?

I recommend using Microsoft's Open XML library (as it doesn't require Excel to be installed).
There are various 3rd party libraries that sit on top of it to make it easier to use; such as ClosedXML or Simple OOXML.

Consider ODBC Excel drivers. If your input file is built by machine AND you do not want anything fancy as output, this should work just fine.

You can try Spreadsheet control to open excel http://www.syncfusion.com/products/user-interface-edition/wpf/spreadsheet
Sample location: http://silverlight.syncfusion.com/samples/WPF/Samples/WPFSampleBrowser/UI/Spreadsheet/Spreadsheet.htm

Save XLS Using Interop Excel

The Current Setup:
So, I can use the following code to save a file as an XLS:
_myWorkbook.SaveAs("FileName.xls", Excel.XlFileFormat.xlWorkbookNormal)
I can also use the following code to save a file as an XLSX (Since I use Office 2010):
_myWorkbook.SaveAs("FileName.xlsx", Excel.XlFileFormat.xlWorkbookDefault)
The Issue:
I have tried (unsuccessfully) to save the file as an XLSX using the following code:
_myWorkbook.SaveAs("FileName.xlsx", Excel.XlFileFormat.xlExcel12)
Why does this not work? Last I checked, Excel 12 was Excel 2007 (a version that supports XLSX). Am I missing something?
(For those interested, I get a This extension can not be used with the selected file type error)

Actually XlFileFormat.xlExcel12 is Excel 12.0 binary format (xlsb) , for OpenXML (i.e. xlsx) you need to use XlFileFormat.xlOpenXMLWorkbook enum value.
For a complete reference see here.

CSV, Excel, PDF Report Generator for Mysql, .net 4.0

I am in a strange juncture, where I need to generate reports in Excel, PDF and CSV formats.
I got it working for CSV format. but have no clue on how to generate Excel and PDF reports.
I am sure I dont have to re-invent the wheel to do all this stuff.
I can not use SSRS etc, as they are only for SQL Server.
Could some one please suggest a good reporting tool for Mysql?

You can use open source ClosedXML, а wrapper around OpenXML to conveniently generate xlsx files - i.e. Office 2007+ format Excel files. Can't help with PDF, but you can search codeplex for it too.

Converting HTML table to excel sheet using C#

I have html table in Microsoft.Office.Interop.Outlook.MailItem body and I just need to fill excel sheet with this table using C# for desktop application. Could any one help me in this regard. Thanks

A quick and dirty way:
File.WriteAllText(#"C:\Temp\Table.xls", mailItem.Body);
Excel will open it even though the file does not contain a valid xls document

You can also use HtmlAgilityPack to parse the e-mail and EPPlus to write to Excel (only 2007/2010 xlsx version).