问个简单c#语法问题

 /// <summary>
        /// 
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expand"></param>
        /// <param name="key">键</param>
        /// <param name="t">缺省值</param>
        /// <returns></returns>
        public static T QueryString<T>(this HttpRequest expand, string key, T t)
        {
            return HttpContext.Current.Request.QueryString[key] != null ? (T)HttpContext.Current.Request.QueryString[key] : t;
        }