now is ok

I just understood your first reply. So, now I put it this way and works fine without touching the target app source code :

Assembly assembly = loadAssembly(assemblyName);

Type t = assembly.GetType(mainClass);

Object obj = Activator.CreateInstance(t);

mainWindow = (Window)obj;

and then used mainWindow as if I were at home. ( mainWindow.Show(), etc)

thank you for your article and support.