为什么这个错误，“序列不包含任何元素”，怎么回事？ [英] Why is this error, 'Sequence contains no elements', happening?

问题描述

我收到一个无效操作异常，堆栈楼下。我想这是因为 db.Responses.Where（Y =＆GT; y.ResponseId.Equals（item.ResponseId））第一（）; 没有返回任何结果。我查了响应数据和userResponseDetails有ResponseId，我也只是用硬codeD值。我也知道，调用本该语句添加响应排，这个功能应该被调用。 （这是前工作一个月左右，我不记得任何改变，将打破这个）

  [出现InvalidOperationException：序列不包含任何元素]
   System.Linq.Enumerable.First（IEnumerable`1源）+269
   System.Data.Objects.ELinq.ObjectQueryProvider＆放大器; LT; GetElementFunction和放大器; GT; b__0（IEnumerable`1序列）+41
   System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle（IEnumerable`1查询，防爆pression queryRoot）+59
   System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute（前pression前pression）+133
   System.Data.Entity.Internal.Linq.DbQueryProvider.Execute（前pression前pression）+87
   System.Linq.Queryable.First（IQueryable`1源）+251
   InSight.Controllers.ForecasterController.userResponseDetails（List`1 userResponseDetails）1039
 

下面是违规code。

  [HttpPost]
    公共JsonResult userResponseDetails（列表＆LT; ResponseDetailsPartial＆GT; userResponseDetails）
    {        的foreach（在userResponseDetails ResponseDetailsPartial项）
        {
            ResponseDetails临时=新ResponseDetails（）;
            temp.ResponseId = item.ResponseId;
            temp.ResponseDetailVal = item.ResponseDetailVal;
            temp.QuestioChoicesId = item.QuestioChoicesId;
            temp.Response = db.Responses
                  。凡（Y =＆GT; y.ResponseId.Equals（item.ResponseId））第一（）。
            temp.QuestionChoice = db.QuestionChoices
                   。凡（X =＆GT; x.QuestionChoicesId.Equals（item.QuestioChoicesId））第一（）。
          db.ResponseDetails.Add（临时）;
        }
        db.SaveChanges（）;        返回JSON（新{ResponseDetailsId = userResponseDetails}，JsonRequestBehavior.AllowGet）;
    }
 

这是调用这一特定的行动AJAX的：

  $。阿贾克斯（{
         键入：POST，
         网址：'/预报/ userResponseDetails /'，
         数据：JSON.stringify（rdetail）
         数据类型：JSON，
         的contentType：应用/ JSON，
     }）
 

和这是rdetail已经strigified后：

  [{ResponseId：118，ResponseDetailVal0.36，QuestioChoicesId：空}]
 

解决方案

再次检查。如果必须使用调试器。我的猜测是，在一些userResponseDetails项目该查询发现没有元素：

 。凡（Y =＆GT; y.ResponseId.Equals（item.ResponseId））
 

所以你不能叫

 。首先（）
 

就可以了。也许尝试

  .FirstOrDefault（）
 

如果它解决了问题（就像一个测试）。

I am getting an Invalid Operation Exception, the stack is down below. I think it is because db.Responses.Where(y => y.ResponseId.Equals(item.ResponseId)).First(); is not returning any results. I checked the response data and the userResponseDetails has a ResponseId, I also just used a hard coded value. I also know that the statement that calls this one is adding the Responses row that this function should be calling. (This was working about a month ago and I don't remember changing anything that would break this)

[InvalidOperationException: Sequence contains no elements]
   System.Linq.Enumerable.First(IEnumerable`1 source) +269
   System.Data.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__0(IEnumerable`1 sequence) +41
   System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle(IEnumerable`1 query, Expression queryRoot) +59
   System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute(Expression expression) +133
   System.Data.Entity.Internal.Linq.DbQueryProvider.Execute(Expression expression) +87
   System.Linq.Queryable.First(IQueryable`1 source) +251
   InSight.Controllers.ForecasterController.userResponseDetails(List`1 userResponseDetails) +1039

Here is the offending code.

    [HttpPost]
    public JsonResult userResponseDetails(List<ResponseDetailsPartial> userResponseDetails)
    {

        foreach (ResponseDetailsPartial item in userResponseDetails)
        {
            ResponseDetails temp = new ResponseDetails();
            temp.ResponseId = item.ResponseId;
            temp.ResponseDetailVal = item.ResponseDetailVal;
            temp.QuestioChoicesId = item.QuestioChoicesId;
            temp.Response = db.Responses
                  .Where(y => y.ResponseId.Equals(item.ResponseId)).First();
            temp.QuestionChoice = db.QuestionChoices
                   .Where(x => x.QuestionChoicesId.Equals(item.QuestioChoicesId)).First();
          db.ResponseDetails.Add(temp);
        }
        db.SaveChanges();

        return Json(new { ResponseDetailsId = userResponseDetails }, JsonRequestBehavior.AllowGet);
    }

This is the AJAX that calls this specific action:

$.ajax({
         type: "POST",
         url: '/Forecaster/userResponseDetails/',
         data: JSON.stringify(rdetail),
         dataType: 'json',
         contentType: 'application/json',
     })

and this is rdetail after it has been strigified:

[{"ResponseId":118,"ResponseDetailVal":0.36,"QuestioChoicesId":null}] 

解决方案

Check again. Use debugger if must. My guess is that for some item in userResponseDetails this query finds no elements:

.Where(y => y.ResponseId.Equals(item.ResponseId))

so you can't call

.First()

on it. Maybe try

.FirstOrDefault()

if it solves the issue (just as a test).

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