How the service contract class get the host application config dat

In pratical , the host application and service contract class is seperated,
if the service contract class need to set some variable from config file
using Properties.Settinngs.Default.[KeyName] then it need to have
ServiceClass.dll.app.config. Host application have its own Host.app.config.

So how to consolidate all configuration data for both into one which is only
Host.app.config? If can, how to use the Properties.Settings.Default to
retrieve from service contract class?

Thanks!