[WebInvoke(Method = "POST", ResponseFormat = WebMessageFormat.Json,
           RequestFormat = WebMessageFormat.Json, UriTemplate = "login", BodyStyle = WebMessageBodyStyle.WrappedRequest)]
[OperationContract]
string login(string username, string password);

public string login(string username, string password)
{
   string str = String.Empty;
   if (username == "admin" && password == "admin")
       str = "Success";
   else
       str = "Invalid";
   return str;
}