true if the directory exists in the virtual file system; otherwise, false.
Override the VirtualPathProvider.DirectoryExists(string) method to indicate to the compilation system that the resource represented by virtualDir exists in the virtual file system provided by this System.Web.Hosting.VirtualPathProvider instance.
If your custom System.Web.Hosting.VirtualPathProvider class does not support directories, the VirtualPathProvider.DirectoryExists(string) method should return false.
If your virtual file system will contain themes for the Web site (by creating a virtual App_Themes directory), your custom System.Web.Hosting.VirtualPathProvider class must support directories.