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 think you want to reduce the size of your excel file. If yes then you can do it by saving it as xlsb i.e., Excel Binary Workbook format. Further, you can reduce your file size by deleting all the blank cells.