c# - Error while reading ConnectionString from App.config file -

i have app.config file:

<?xml version="1.0" encoding="utf-8" ?> <configuration>   <connectionstrings>     <add name="acwconnectionstring" connectionstring="data source=ashish-pc\\sqlexpress;initial catalog=acw;user id=ash159;password=password"/>   </connectionstrings> </configuration> 

which using here:

string connstring = configurationmanager.connectionstrings["acwconnectionstring"].tostring(); 

here getting error 'nullreferenceexception unhandled'..
why getting error?
thankyou!!

tried these getting same error:

 string connstring = configurationmanager.connectionstrings["acwconnectionstring"].connectionstring;  string connstring = configurationmanager.connectionstrings["acwconnectionstring"].connectionstring.tostring(); 

note: have added system.configuration , set reference it.
know have 3 projects(admin,employee,database) in single project(abc). have added reference of database admin , employee. , made database class in 2 projects wherever needed connection.
database class follows:

 public class database      {             sqlconnection connection;             string connstring = configurationmanager.connectionstrings["acwconnectionstring"].tostring();             public sqlconnection connect()             {                 connection=new sqlconnection(connstring);                 return connection;             }         } 

it in database project have added app.config file.

try this:-

string connstring = configurationmanager.connectionstrings["acwconnectionstring"].connectionstring;