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So I know that iQueryables are translated into SQL statements and thus cannot handle all possible methods that you might put into a where clause.

But this is what I'm trying to do:

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

And reading post like these below, I'm comforted that EF5 should be able to translate this.
Getting Entities whose keys match list(or array) of ids
Using LINQ To Query Int Ids From An Array

However, I'm getting this error:

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

And googling this error does not give me much help.

I have also tried 

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.

and 

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.

I'm stuck and my brain is getting mushy beyond the possibility for any type of graceful recovery. Can anyone kindly save me?

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Alex
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1 Answers1

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

should work, if alreadySelectedIs is not null

you can do a null check inside or before your query :

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

(which can be rewritten, depending if you want all or nothing if alreadySelectedIds is null)

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

or

//return nothing if null
x => alreadySelectedIds != null  && alrreadySelectedIds.Contains(x.Id)
Raphaël Althaus
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    It's quite fantastic how a man can struggle in vain with a simple problem, just because the path to correct understanding of the problem is lost among despair and intellectual panic. But what can a man do, but being a man? – Alex Sep 04 '13 at 15:54
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    @Alex too much philosophy for computer science ;) But... indeed ! – Raphaël Althaus Sep 04 '13 at 15:56