How to implement Authentication at service level in WCF

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WCF

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.