[ServiceContract]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
// NOTE: If the service is renamed, remember to update the global.asax.cs file
public class MessageService
{
    [WebGet(UriTemplate = "/Method1?a={var1}&b={var2}")]
    [CompositeOperationBehavior("MessageService")]
    public Message GetMessage(string var1, string var2)
    {
        return WebOperationContext.Current.CreateTextResponse(var1 + var2);
    }
}
 
public class MessageServiceBehaviors
{
    [ExportOperationBehavior(Group = "MessageService")]
    public IOperationBehavior Required
    {
        get { return new RequiredParameterBehaviorAttribute("var2"); }
    }
}