Free compression library for C# which supports 7zip (LZMA)

7-zip has a free source code, public domain C# SDK for the LZMA algorithm:

http://www.7-zip.org/sdk.html

There is a good article written by Peter Bromberg:

7Zip (LZMA) In-Memory Compression with C#

Shows a simple C# SevenZip.Compression.Lzma.SevenZipHelper class to enable high-compression of in-memory objects with C#. Adds a new 7Zip format LZMA Cookie Compression class to the previously published CookieCompression utility.

You may try SevenZipSharp