SharePoint 2010, Sandboxed Solution Limitation
When a sandboxed solution is deployed, the array of SharePoint functionality available to it is limited to help reduce any security vulnerabilities it may have. Some of these limitations include the following:
Sandboxed solutions have a restricted subset of deployable solution elements available to them. Potentially vulnerable SharePoint project templates, such as site definitions and workflows, are not available.
SharePoint runs sandboxed solution code in a process (SPUCWorkerProcess.exe) separate from the main IIS application pool (w3wp.exe) process.
Mapped folders cannot be added to the project.
Types in the Microsoft SharePoint Server 2010 assembly Microsoft.Office.Server cannot be used in sandboxed solutions. Also, only types in the Microsoft SharePoint Foundation 2010 assembly Microsoft.SharePoint can be used in sandboxed solutions.
It is important to note that specifying a SharePoint solution as a sandboxed solution has no affect on SharePoint server; it only determines how the SharePoint project is deployed to SharePoint from Visual Studio and what assemblies it binds to. It does not affect the generated .wsp file, and the .wsp file has no data that directly correlates to the Sandboxed Solution property.