c# - IQueryable for entities .Where(属性在本地数组中)

Question

所以我知道 iQueryables 被翻译成 SQL 语句，因此无法处理您可能放入 where 子句中的所有可能方法。

但这就是我想要做的:

int[] alreadySelectedIds = ...
var subjects = Entities.NewInstance.Subjects.Where(x => Array.IndexOf(alreadySelectedIds, x.Id) == -1).ToList();

阅读下面这些帖子，我很欣慰 EF5 应该能够翻译它。
Getting Entities whose keys match list(or array) of ids
Using LINQ To Query Int Ids From An Array

但是，我收到了这个错误:

LINQ to Entities does not recognize the method 'Int32 IndexOf[Int32](Int32[], Int32)' method, and this method cannot be translated into a store expression.

用谷歌搜索这个错误并没有给我太多帮助。

我也试过

var newSubjects = Entities.NewInstance.Subjects.Where(x => alreadySelectedIds.Contains(x.Id)).ToList();

Unable to create a null constant value of type 'System.Int32[]'. Only entity types, enumeration types or primitive types are supported in this context.

和

List<int> alreadySelectedIds = ...

Unable to create a null constant value of type 'System.Collections.Generic.List`1'. Only entity types, enumeration types or primitive types are supported in this context.

我被困住了，我的大脑变得糊涂，无法进行任何类型的优雅恢复。有好心人救救我吗？

Answer 1

Entities.NewInstance.Subjects.Where(x => alreadySelectedIds.Contains(x.Id)).ToList();

应该工作，如果 alreadySelectedIs 不为空

您可以在查询内部或之前进行空检查:

Entities.NewInstance.Subjects.Where(x => alreadySelectedIds == null 
                                         ? true // or false 
                                         : alreadySelectedIds.Contains(x.Id)
                                    ).ToList();

(如果 alreadySelectedIds 为 null，则可以重写，具体取决于您是想要全部还是什么都不想要)

//return all if null
x => alreadySelectedIds == null || alreadySelectedIds.Contains(x.Id)

或

//return nothing if null
x => alreadySelectedIds != null  && alrreadySelectedIds.Contains(x.Id)