The Windows Live Messenger Library is a client-side library for building Web-based applications that integrate with Windows Live™ Messenger. With the Windows Live Messenger Library, developers can build presence-enabled instant messaging applications that Web site visitors can use to interact with each other and Windows Live Messenger client users.
The Windows Live Messenger Library is written in C# and compiled into JavaScript using Script#. Messenger Library applications can be written either in C# (with Script#) or JavaScript.
The Messenger Library can be used by Web sites hosted in any domain and works by using cross-site API calls to the Windows Live Messenger service.

Documentation Map
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What's New in the Windows Live Messenger Library
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Summarizes new features and changes introduced in the current version
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Developing with the Windows Live Messenger Library
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Covers the fundamentals of developing applications that use the Windows Live Messenger Library, including step-by-step instructions for common tasks and programming conventions unique to the Windows Live Messenger Library.
The use of the Windows Live Messenger Library is governed by the Terms of Use.