referencing a mdiframe in a child-dll that is in a second project

Hi, I just began learning c#, so I hope I am giving enough info with my question. I created a solution with two projects, in one I have a Mdi frame (with one namespace: boekhoud, and one class: frmContainer), in the other I have a child (with one namespace: ClientOnderhoud, and one class: frmClient). I set the parent-childrelation with frmClient child = new frmClient(); child.MdiParent = this; child.Show(); This compiles allright, so I can reference frmClient in my parent. Now I would like to reference frmContainer in my child, I do this with ((frmContainer)this.MdiParent.MyConnection).Open(); It gives me the following error: The type or namespace name 'frmContainer' could not be found (are you missing a using directive or an assembly reference?) What should I do to let this work? regards, Stefan.