Sometimes you have to split your code into different assemblies. For example when I created a custom Admin Page which inherits from WebAdminPageBase (Microsoft.SharePoint.ApplicationPages). The problem with Microsoft.SharePoint.ApplicationPages is that it's not deployed to the GAC. When compiling you will get the following message: “CS0122: 'foo.bar.x' is inaccessible due to its protection level” Now you have only one choice: make x public. Or maybe not?