LINQ to Entities无法识别方法'System.String ToString（Int32）'

Question

Hi I am using a linq query which is throwing the error LINQ to Entities does not recognize the method 'System.String ToString(Int32)' method, and this method cannot be translated into a store expression.

        List<string> resultMap = (from item in mapResult
                                  select Convert.ToString(item.ResultDE)).ToList();

Error is throwing in this below statement

        List<Result_DE> resultList = (from result in db.Result_DE
                                      where result.IsActive == "1"
                                      && resultMap.Contains(Convert.ToString(Convert.ToInt32(result.ID)))
                                      select result).ToList();

please tell me the proper way of writing this query.

Answer 1

You cannot use these conversion functions in a LINQ to Entities statement, they cannot be translated to SQL, you need to do the conversions in memory. But I don't think you need to do that at all.

If you were just using the resultMap to get your resultList , filtered by Results of which the Id is present in mapResult , do the following:

var resultList = db.Result_DE
    .Where(r => r.IsActive == "1" && mapResult.Any(mr => mr.ResultDE == r.ID));
    .ToList();

If mapResult is an in-memory collection, instead of an IQueryable that is attached to the db context, you need to do the following:

var resultIds = mapResult.Select(mr => mr.ResultDE).ToList();
var resultList = db.Result_DE
    .Where(r => r.IsActive == "1" && resultIds.Contains(r.ID));
    .ToList();

Answer 2

在调用任何方法（例如ToString（））之前，需要使用AsEnumerable（）将LINQ转换为Object。

Answer 3

if your item.ResultDE and result.ID is variable type of Int32, why don directly create a List<Int32> ?

List<Int32> resultMap = (from item in mapResult
                              select item.ResultDE).ToList<Int32>();

List<Result_DE> resultList = (from result in db.Result_DE
                                  where result.IsActive == "1"
                                  && resultMap.Contains(result.ID)
                                  select result).ToList<Result_DE>();

Answer 4

Use SqlFunctions.StringConvert instead of Convert.ToString.

A similar question was asked and answered here