asp.net-mvc-3 - MVC 3 - 如何知道查询是否返回 0 行

Question

可能很容易，但我无法解决它。如果数据库中不存在请求的 id，我想将用户重定向到错误页面。我的代码：

public ActionResult Details(int id)
    {

        DetailsAdViewModel DAVM = new DetailsAdViewModel();
        DAVM.Ad = db.Ads.Include("Images").Where(a => a.AdId == id).First();
        DAVM.FirstImage = db.Images.Where(a => a.AdId == id).OrderBy(a => a.ImageId).Take(1);

        // make sure the ad isn't deleted or that it really exists
        if (DAVM.Ad == null)
        {
            return RedirectToAction("ShowError", "Error", new { errorCode = "adDeleted" });
        }

        return View(DAVM);
    }

这不起作用，如果我输入错误的 ID，则会出现服务器错误。

Answer 1

使用.FirstOrDefault()而不是.First(). 如果没有找到记录，这将返回 null 而不是抛出异常：

DAVM.Ad = db.Ads.Include("Images").FirstOrDefault(a => a.AdId == id);

现在您可以检查是否DAVM.Ad为空：

if (DAVM.Ad == null)
{
    return RedirectToAction("ShowError", "Error", new { errorCode = "adDeleted" });
}

Answer 2

找到了解决方案：

公共动作结果详细信息（int id）{

        if (db.Ads.Any(a => a.AdId == id))
        {
            DetailsAdViewModel DAVM = new DetailsAdViewModel();
            DAVM.Ad = db.Ads.Include("Images").Where(a => a.AdId == id).First();
            DAVM.FirstImage = db.Images.Where(a => a.AdId == id).OrderBy(a => a.ImageId).Take(1);

            return View(DAVM);
        }
        else
        {
            return RedirectToAction("ShowError", "Error", new { errorCode = "adDeleted" });
        }
    }