c# - how access connectionstring in a web.config in other project from a WCF -

i'm having difficulty access connectionstrings in web.config wcf "web services"

cenario: solution inside visual studio 2010 2 web projects, placement , wcfplacement. web.config has connectionstings inside placement project.

the wcfplacement not have web.config can see in picture, when build it, generates one.

from placement project can access connection strings using configurationmanager.connectionstrings["xx"].connectionstring; - ok.

but if i'm inside service, gives me error when try access, forcing me have connection string declared inside each class want database access, , that's not good.

how can access connectionstrings in th web.config inside placement project wcf ?

i tried:

sqlconnection conn = new sqlconnection(configurationmanager.connectionstrings["webv"].connectionstring); 

but gives me error: system.nullreferenceexception: object reference not set instance of object. @ wcfplacement.prospect.dologin(string login, string senha)

tried inside class:

system.configuration.configuration rootwebconfig =             system.web.configuration.webconfigurationmanager.openwebconfiguration("/placement");             system.configuration.connectionstringsettings connstring;             if (rootwebconfig.connectionstrings.connectionstrings.count > 0)             {                 connstring =                     rootwebconfig.connectionstrings.connectionstrings["northwindconnectionstring"];                 if (connstring != null)                     console.writeline("northwind connection string = \"{0}\"",                         connstring.connectionstring);                 else                     console.writeline("no northwind connection string");             } 

but error. error 1 type or namespace name 'configuration' not exist in namespace 'system.web' (are missing assembly reference?)

you have add reference system.configuration namespace.

https://msdn.microsoft.com/en-us/library/system.configuration(v=vs.110).aspx