The ConnectionString property has not been initialized

There are many sample connectionString methods, try it

(i) 

System.Web.Configuration.WebConfigurationManager.ConnectionStrings("Blah").ConnectionString

(ii)

Dim connString As String = System.Configuration.ConfigurationManager.ConnectionStrings("yourConnectionString").ConnectionString

(iii)

Dim MyConnection As SqlConnection = New SqlConnection(ConfigurationManager.AppSettings("CONN_DATAConv"))

Dim ProcConnection As SqlConnection = New SqlConnection(ConfigurationManager.AppSettings("CONN_DATAConv"))

Dim myCommand As New SqlCommand(CommandText, MyConnection)

MyConnection.Open()

ProcConnection.Open()

(iv) 

string connectionString = ConfigurationSettings.AppSettings"AsynchronousSocketListener.Properties.Settings.infoConnectionString"];

SqlConnection conn = new SqlConnection(connectionString);

(v)

string connectionString = ConfigurationSettings.AppSettings["ConnectionString"];

(vi)

ConnectionString = DBConnectionString.GetConnectionString(ConfigurationManager.AppSettings("ConnectionString"), ConfigurationManager.AppSettings("ConnectionStringUsername"), ConfigurationManager.AppSettings("ConnectionStringPassword"))

(vii)

MySQLHelper.ExecuteNonQuery(ConfigurationManager.ConnectionStrings["MyDB"].ConnectionString,CommandType.Text,sqlQuery,sqlParams);

(ii) 

SqlConnection myCon = new SqlConnection(ConfigurationManager.ConnectionStrings[nameOfString].ConnectionString;

(iii) 

SqlDataAdapter adapter = new SqlDataAdapter(sql, ConfigurationManager.ConnectionStrings["FADB_ConnectionString"].ConnectionString); 

DataTable data = new DataTable(); 

DataSet ds = new DataSet();