This project is the .NET version of POI Java project at http://poi.apache.org/. POI is an open source project which can help you read/write Office 2003/2007 files.
The source code in GitHub is a clone of the SVN http://npoi.googlecode.com/svn/trunk/.
a. It's totally free to use
b. Cover most features of Excel (cell style, data format, formula and so on)
c. Professional support service (even 24*7) from NPOI team (not free)
d. Support xls, xlsx, docx.
e. Designed to be interface-oriented (take a look at NPOI.SS namespace)
f. Support not only export but also import
g. .Net 2.0 based even for xlsx and docx (though we also support .NET 4.0)
h. Real successful cases all over the world
i. huge amout of basic examples
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VS2010 with .NET 4.0 runtime
VS2005 or VS2008 with .NET 2.0 Runtime (SP1)
vs2003 with .NET 1.1 (Obselete, last version is 1.2.1. No further support)