uSING crm frAMEwORK eRROR

Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

USING CRM METHOD RETRIEVE

 var crm = CrmServices.Create(VPNEntity);
 var vpn = CrmServices.Retrieve("lead", crm, new ColumnSet("subject"));