Best and fast way to export to excel by c# code in Sharepoint 2010 ?

It would probably be quicker to run a CAML query and write the output from that directly to file.

e.g.: https://msdn.microsoft.com/en-us/library/office/ee534956%28v=office.14%29.aspx

The only issue is CAML is a bit of a pain, so use a tool such as CAML Designer to build your query, then use that in your code.

Another alternative is to use SSIS to extract Sharpeoint List data to a database, then extract or report on whatever you need from that.  I've done that successfully on SP2010 and its pretty easy to do if you know SSIS.  The advantage of this is if you have regular reporting requirements it's straightforward to schedule a package and rebuild reports and data overnight for example in a data warehousing scenario.

Hi,

 I can use to CAML query to grab data but my question was to export to excel very quickly with good performance.

Use CAML to grab the data, iterate over the resultset, and write it to a flat .csv file usng FileIO.  This will remove all the overhead of binding to a gridview.

Hi,

 Do we have any sample code or example to write it to flat .csv ?

If you don't like that then as I say you could use a report in SSRS to extract the data.  I think modern versions of this allow querying SharePoint lists as a datasource, although I have never done this:
https://msdn.microsoft.com/en-us/library/ee633650.aspx

This is a code free solution, and you could even use the report scheduler to to run this every day and email it to you for example.  You can extract to Excel from the report viewer front end.

Hi,
from another question I answered a couple of days ago I found a really neat JavaScript library which can create Excel files in JavaScript.

The open source project can be found here:
https://github.com/stephenliberty/excel-builder.js/tree/master

Unfortunately their main web site has currently some issues, but using Google Search and there the cached site, you still can get an idea how to use.
You simply have to pass the data Array, but you can also manipulate data types, create multiple Sheets in one workbook ...

I will definitly use it in further projects.

HTH
Rainer

Thanks