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 xls, doc, ppt files. It has a wide application.
For example, you can use it to
a. generate a Excel report without Microsoft Office suite installed on your server and more efficient than call Microsoft Excel ActiveX at background;
b. extract text from Office documents to help you implement full-text indexing feature (most of time this feature is used to create search engines).
c. extract images from Office documents
d. generate Excel sheets that contains formulas
Advantage of NPOI
a. It's totally free to use
b. Cover most features of Excel (cell style, data format, formula and so on)
c. Support xls, xlsx, docx.
d. Designed to be interface-oriented (take a look at NPOI.SS namespace)
e. Support not only export but also import
f. .Net 2.0 based even for xlsx and docx (though we also support .NET 4.0)
g. Successful cases from all over the world
h. huge amout of basic examples
i. No dependency on isolated storage
Comments from NPOI users
NPOI on SNS
VS2010 with .NET 4.0 runtime
VS2005 or VS2008 with .NET 2.0 Runtime (SP1)
vs2003 with .NET 1.1
medium trust environment in ASP.NET