求高手解释代码

Question

public ActionResult Details(int page, int rows, FormCollection collection)        {            string JobCode = collection["JobCode"] ?? "";            string JobName = collection["JobName"] ?? "";            string IsActive = collection["IsActive"] ?? "";            var job = StudentService.GetDetails(page, rows, JobCode, JobName, IsActive);            return Json(job, "text", JsonRequestBehavior.AllowGet);        } 谁能解释下两个问号是什么意思呀？

Answer 1

说实话，没看明白

Answer 2

楼上和楼下已经解释的很清楚了，可以看看

Answer 3

?? 意思就是 如果为空就用后面的值。