I use project->properties->building settings->project dependencies to make a project depend on b project.
It works ok.
I save the project (or the workspace). When I close the project(or workspace), and reopen it , the dependencies setting disappeared.
The c::b donnot save this setting .
It can but you have to save the workspace and later reopen it, not the separate projects.
The dependency info is stored in .workspace file.
Thanks , it is ok now.