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I need a standalone .net dll to process openxml spreadsheet formulas realtime

I am writing an application that will live on a server I will not have access to.
My task is to build a web interface that will encapsulate some EXTENSIVE (200+ formulas) logic based in an Excel spreadsheet.
I will not be able to install any software on this server.
I do not want to recode all the formulas in C#.
What I would like is to use a standalone dll that will allow me to open an OpenXML formatted spreadsheet file, change the input cell values, and then extract the end values from cells containing formulas which have run on the new data.
Is there anything out there other than Excel interop (which requires Excel be installed) that will actually compute the formulas realtime?

Since you want to work with OpenXML files, you can use OpenXML SDK 2.0, which is available here: http://www.microsoft.com/download/en/details.aspx?id=5124. You should install it on your developer environment and it helps you to read and modify contents of OpenXML files in your .NET application.
I haven't tried to work with formulas using this SDK, but I suppose it's possible, however, it may require some work (like writing a parser) to recalculate the formulas with you program.
There is, however, a workaround for this, but I'm not sure if it's acceptable for you. You can make Excel reevaluate all the formulas itself when the file is opened by a user. You can find some information about it here: OpenXML SDK: Make Excel recalculate formula.
I also recommend you to use OpenXML SDK 2.0. Productivity Tool, which is a part of the SDK. It allows you to view the contents of an OpenXML file and even show you the c# code required to recreate it programmatically, so it can be really helpful.

C# output to Excel spreadsheet, using Excel 2000+

I'm writing a program that reads a text file, extracts information, and outputs it to a template Excel spreadsheet that already exists.
I've managed to do this on my computer using the Microsoft.Office.Interop.Excel reference and its related methods, and it works fine. I have Excel 2010. However the computers that this program will be used on mostly have either Excel 2000 or Excel 2003, and it won't work on them.
Does anyone know a way to make a program target all versions of Excel from 2000 upwards?
Cheers,
Greg

If your needs are simple and you don't have $900 for Aspose.Cells to throw around, you can do any of the following:
Use NPOI to read, inject data into, and export your template.
Create a basic HTML file with a table and just named it *.xls. You can save your template in Excel as HTML and replace bits and pieces to insert your data.
Create an XML file using Office 2002/2003 XML format, it's pretty straightforward (caveat: can't be read in Excel 2000). As above, you can save your template in XML Spreadsheet format, read it in, and do some simple stuff to inject your data.

You really need to target 2000 or under. 2010 and 2003 will open a 2000 format document whereas 2000 will not open a 2010 document. Office has a single format for 97-2000 and that's what you need to create to make everybody happy.
Interop depends on the version you have installed and I personally have dodged using interop due to its "unmanaged" nature (and it seems to love file locks).
If you want hassle free and extremely fast/powerful creation of Excel documents, you really can not do better than Aspose.Cells in my opinion.
Find it here.

Reading Excel Cells using C#

What's the way to open a excel workbook and to read excel cells?

In this thread you will get details how to open and handle excel through C#.

Use Visual Studio Tools for Office (VSTO) to extend Office products with your code and here's a good tutorial to get you started in Excel programming

There is an open source .NET library called Koogra for reading Excel files, both BIFF (older Excel) and XSLX (newer Excel). Otherwise you can use Excel automation, but I would avoid that if possible.

You can also use open xml sdk to read (and write) excel files (in open xml format off course). There is an msdn article describing how to use the open xml sdk.
A big advantage is that you don't have to install excel on the server (or client) to read excel files.

What is the best practice to create a Excel 2007 Workbook

I want to know what is the best practice to create a Excel 2007 Workbook using C#, with its datasource being a raw flat file or a table in database.

You can use
"Open XML SDK 2.0 for Microsoft Office"
It's more comfortable than harcore manually hacking OpenXML spec. There are .NET strongly typed wrapper classes so it's not hard to create a simple sheet. You don't need any interop and msoffice installed and it's safe for server soluitions - there are only a few dlls which you can ship in your solution.
I did mail-merge solution and it wasn't so scary.
But as always, when it's possible, I'm prefering plain csv format.

I personally like creating CSV's, which can be opened directly in Excel. It's a lot less work than trying to hack the Office Open XML specification, and you don't need COM interop to Excel (which requires a copy of Excel to work).

You can use the Office Primary Interop Assemblies to completely automate Excel 2007, and create the workbook from within C#.
This gives you the most control, as you have complete control over how you map from DB or flat file -> Excel workbook/worksheet.

I am using
http://www.leniel.net/2009/07/creating-excel-spreadsheets-xls-xlsx-c.html
for creating excel . Seems good so far.

How can I programmatically create, read, write an excel without having office installed?

I'm confused as hell with all the bazillion ways to read/write/create excel files. VSTO, OLEDB, etc, but they all seem to have the requirement that office must be installed.
Here is my situation: I need to develop an app which will take an excel file as input, do some calculations and create a new excel file which will basically be a modification of the first excel file. All with the constraint that the machine that runs this may not have office installed. (Don't ask why...)
I need to support all excel formats. The only saving grace is that the formats spreadsheets themselves are really simple. Just a bunch of columns and values, nothing fancy. And unfortunately no CSV as the end user might not even know what a CSV file is.

write your excel in HTML table format:
<html>
<body>
<table>
<tr>
<td style="background-color:#acc3ff">Cell1</td>
<td style="font-weight:bold">Cell2</td>
</tr>
</table>
</body>
</html>
and give your file an xls extension. Excel will convert it automatically

Without Office installed you'll need something designed to understand the Excel binary file format (unless you only want to open Office 2007 .xlsx files).
The best I've found (and that I use) is SpreadsheetGear, which in addition to being .NET native, is much faster and more stable then the COM/OLE solutions (which I've used in the past)

read and write csv files instead. Excel reads them just fine and they're easier to use. If you need to work against .xls files then try having support for OpenOffice as well as Excel. OpenOffice can read and write excel files.

Did you consider way number bazillion and one: using the Open XML SDK? You can retain styles and tweak it to your liking. Anything you can do in an actual file is possible to achieve programatically. The SDK comes with a tool called Document Reflector that shows the underlying XML and even shows LINQ statements that can be used to generate them. That is key to playing around with it, seeing how the changes are made, then recreating that in code.
The only caveat is this will work for the new XML based formats (*.xlsx) not the older versions. There's a slight learning curve but more material is making its way on blogs and other sites.

If cost is not an issue, I'd suggest looking in Aspose's Excel product. I use their Word product and I've been satisfied.
Aspose.Cells

Excel XLSX files "just" XML files - more precisely ZIP files containing several XML files. Just rename a Excel file Test.xslx to Test.zip and open it with your favourit ZIP program. XML schemas are, afaik, standardized and availiable. But I think it might not be that easy to manipulate them only using primitive XML processiing tools and frameworks.

Excel files are in a proprietary format so (afaik) you're not going to be able to do this without having the office interop available. Some third party tools exist (which presumably licence the format from MS?) but I've not used them myself to comment on their usefulness.
I assume that you can't control the base file format, i.e. simple CSV or XML formats aren't going to be possible?

I used to use a very nice library called CarlosAg, which uses Excel XML format. It was great (and Excel recognizes the format), and also incredibly fast. Check it out here.
Oh, as a side note, we used to use this for the very same reason you need it. The servers that generated these files were not able to have Excel installed.

If you cannot work with CSV files as per #RHicke's suggestion, and assuming you are working on a web app, since a desktop app would be guaranteed to have XL installed as per requirements.
I'd say, create your processing app as a webservice, and build an XL addin which will interact with your webservice directly from XL.

For XLSX files, look at using http://www.codeplex.com/ExcelPackage. Otherwise, some paid 3rd party solutions are out there, like the one David suggested.

I can understand the requirement of not having office installed on a server machine.
There are many libraries like aspose being available, some of them requiring license though.
If you are targeting MS Excel formats, then a native, Interoperability library, ACE OLEDB data provider, from Microsoft is available which you can install on a machine and start reading, writing programmatically. You need to define a connection string and commands as per you needs. (Ref: This article #yoursandmyideas)talks about using this library along with setup and troubleshooting information.