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Understanding Sandboxed Solutions

A sandboxed solution is packaged as a web solutions package (.wsp) and deployed to the Solution Gallery of a site collection. Since a sandboxed solution is not allowed to access anything outside the site collection, features deployed as a part of sandboxed solutions cannot be scoped beyond the level of a site collection. Further, only a restricted subset of the SharePoint object model is available to sandboxed solutions.

The most important thing to note when developing sandboxed solutions is that you cannot deploy anything to the file system of servers. This implies that you cannot deploy components such as application pages, user controls, and so on, requiring access to the file system using sandboxed solutions. ...