如何使用linq到实体检索索引处的项目 [英] How do I retrieve an item at an index using linq to entities

问题描述

//大家好，我已经尝试了几种方法。我的目的是从问题库中获取一些具有不同项目的随机问题。所以我根据问题库的大小生成了一些随机索引，例如40，并将它们存储在一个整数数组中。我现在循环遍历数组以检索数组索引处的项目。每当我尝试几个想法时，我最终都会收到此错误。



LINQ to Entities无法识别方法'CBTDLCF.QuestionTable ElementAtOrDefault [QuestionTable]（System.Linq.IQueryable`1 [CBTDLCF.QuestionTable]，Int32 ）'方法，并且此方法无法转换为商店表达式。

//Hello people, I've tried several ways of going about this. My aim is to get some random questions from a question bank with distinct items. So I generated some random indexes based on the size of my question bank say 40 for example and get them stored in an array of integers. I now looped through the array to retrieve an item at the index of the array. I ended up receiving this error each time I try several ideas.

LINQ to Entities does not recognize the method 'CBTDLCF.QuestionTable ElementAtOrDefault[QuestionTable](System.Linq.IQueryable`1[CBTDLCF.QuestionTable], Int32)' method, and this method cannot be translated into a store expression.

//This is what I wrote
public List<QuestionTable> RandomQuestionWIthDistinctElement()
{
     Random randIndex = new Random();
     int totalQuestions = context.QuestionTables.Count();
     int[] randomIndex = new int[totalQuestions];
     List<QuestionTable> d = new List<QuestionTable>();

     for (int i = 0; i < totalQuestions; i++)
     {
           randomIndex[i] = randIndex.Next(1,totalQuestions);
     }

     for (int i = 0; i < randomIndex.Distinct().Count(); i++)
     {
           var element = context.QuestionTables.ElementAt(index);
           d.Add(element);
     }

     return d;
}

//So I make use if the function in my Page_Load like this
//
var result = RandomQuestionWIthDistinctElement().Take(5).ToList();
questionTable = ToDataTable(result);//convert to sql datatable
//I don't know why I'm getting this error

推荐答案

尝试：

Try:

var element = context.QuestionTables.ToList()[index];

基本上，您试图在表中获取特定的行号。没有SQL等价物（至少是一种简单的方法）来实现这一目标。添加 .ToList（）您正在强制立即执行查询并将结果带入内存，特别是 List


这个问题是，如果你的表很大，你会减少很多数据。理想情况下，您应该根据源表中的键或列查询表。

Basically, you are trying to get a specific row number in your table. There is no SQL equivalent (at least a simple way) to get this. Adding the .ToList() you are forcing an immediate execution of the query and bring the results into memory, specifically into a List object. This has a nice easy way to access the data via an index.

The issue with this is, if your table is large, you are bringing down a lot of data. Ideally, you should be querying the table based on a key or column in the source table.

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