Question: I am calling a web service and the data from the web service is in csv format. 
If I try to save data in xls/xlsx, then I get multiple sheets in a workbook.
So, how can I save the data in csv with multipletab/sheets in c#.

I know csv with multiple tabs is not practical, but is there any damn way or any library to save data in csv with multiple tabs/sheet?

Answer: 
> 
> If I try to save data in xls/xlsx, then I get multiple sheets in a workbook.
> 
> 
> 

Your answer is in your question, don't use text/csv (which most certainly can **not** do multiple sheets, it can't even do one sheet; there's no such thing as a sheet in text/csv though there is in how some applications like Excel or Calc choose to import it into a format that does have sheets) but save it as xls, xlsx, ods or another format that does have sheets.

Both XLSX and ODS are much more complicated than text/csv, but are each probably the most straightforward of their respective sets of formats.
I've been using this library for a while now, 

<https://github.com/SheetJS/js-xlsx>

in my projects to import data and structure from formats like: xls(x), csv and xml but you can for sure save in that formats as well (all from client)! 

Hope that can help you,, take a look on online demo, 

<http://oss.sheetjs.com/js-xlsx/>

peek in source code or file an issue on GH? but I think you will have to do most coding on youre own