Built in C# and can be used by all languages the .NET environment supports.
Edit by : IM SDK For .NET Framework/.NET Compact Framework provide connectivity with the MSN Messenger service. The library is built in C# and can therefore be used by all languages the .NET environment supports. Because of a clean natural Object-Oriented approach the library is easy to use and implement. It is designed to be a flexible, robust and lightweight enhancement to any application. Applications of IM SDK For .NET Framework/.NET Compact Framework vary from creating messaging robots to creating custom clients. If your application needs to communicate through the messenger service, IM SDK For .NET Framework/.NET Compact Framework is your best tool.
Features
Send and receive instant messages(Unicode supported) to and from other public IM client via IM Server
hange IM account status
Change IM account nickname
Add and remove other client from contact, Allow and block lists
List,add,remove and modify group
Messenger IM Control & Presence supported
Send and Receive avatar images
Send and Receive file
Send and Receive Shake message
Personal message supported.

new:
[MSN] Upgrade MSN protocol to MSNP18.
[MSN] Signing in from multiple places (mpop) is supported.
[Yahoo] Upgrade Yahoo protocol to YMSG16.
[ALL] Fixed some minor bugs you can free download IM .NET SDK 5.0 now.