Occasionally, our InfoPath Forms Services solutions require a direct submission to a SharePoint list using a Web service. In order to use SPSite, SPWeb, SPFolder, SPFile, and other classes, your Web service will need a reference to the Microsoft.SharePoint namespace, which provides the types and members for working with SharePoint sites. But where is the corresponding Microsoft.SharePoint.dll?
We get this question a lot, and it has also made its way into numerous SharePoint development forums. For Office SharePoint Server 2007 solutions, the DLL can be found in <DRIVE>:Program FilesCommon FilesMicrosoft Sharedweb server extensions12ISAPI on the SharePoint server.
NOTE: You can create a development environment separate from the SharePoint server that includes a reference to the DLL and allows you to compile the Web service code. However, the Web service can run only on the SharePoint server machine.