How to implement Authentication at service level in WCF

Question

Can someone assit me how to implement Authentication at service level in WCF ?
In below example i want to apply Authentication on class "User" and method "Test()"
 
[ServiceContract]
public interface IUser
{
[OpeartionContract]
string Test();
}
 
//User.svc file
Public class User : IUser
{
public string Test()
{return "";}
 
}
 
If possible please suggest any exmaple.