"The LINQ expression node type 'Invoke' is not supported in LINQ to Entities" - stumped!

Question

In my EF later, I'm trying to pass in an anonymous function to be used as part of my Linq query. The function would pass in an INT and return a BOOL (u.RelationTypeId is an INT). Below is a simplified version of my function:

public IEnumerable<UserBandRelation> GetBandRelationsByUser(Func<int, bool> relation)
{
    using (var ctx = new OpenGroovesEntities())
    {
        Expression<Func<UsersBand, bool>> predicate = (u) => relation(u.RelationTypeId);

        var relations = ctx.UsersBands.Where(predicate);

        // mapping, other stuff, back to business layer
        return relations.ToList();
    }
}

However, I get the error stated above. It seems like I'm going everything correct by building a predicate from the function. Any ideas? Thanks.

Equation

Answer 1

You're trying to pass an arbitrary .NET function in... how could the entity framework hope to translate that into SQL? You can change it to take an Expression<Func<int, bool>> instead, and build the Where clause from that, although it won't be particularly easy, because you'll need to rewrite the expression with a different parameter expression (i.e. replacing whatever parameter expression is in the original expression tree with the expression of calling u.RelationTypeId).

To be honest, for the sake of just specifying u.RelationTypeId in the lambda expression that you use to create the expression tree to pass into the method, you'd be better off just using:

public IEnumerable<UserBandRelation> GetBandRelationsByUser(
    Expression<Func<UsersBand, bool>> predicate)
{
    using (var ctx = new OpenGroovesEntities())
    {
        var relations = ctx.UsersBands.Where(predicate);

        // mapping, other stuff, back to business layer
        return relations.ToList();
    }
}

Marquismarquisate

Thanks. That's sort of what I thought, but my problem is that UserBandRelation is a model while UsersBand is my entity model. I use automapper to map these. Automapper has a problem if I try something crazy like mapping the expressions. Is there any way around this, in order to keep my entities separate? Thanks. – Equation

@Ryan: As I say, you could try to go about doing expression tree rewriting. It's just not something I've tried... I also haven't used Automapper, so I'm not quite sure where that comes into it. – Marquismarquisate

@Ryan somewhere here on SO is a sample I wrote that will happen flatten Expression.Invoke for the purpose of allowing EF to work with sub-expressions (something L2S supports out-of-the-box). It still can't work with delegates, of course - In just saying that a rewriter is here somewhere... – Lee

@Ryan here, in fact - just enable "inline" – Lee

@Ryan @Histone There is also LinqKit, which incorporates many features needed for manipulating expressions. – Sidonie

I second Chris's suggestion of LinqKit => it makes chaining Expression<Func<YourType, bool>> predicates together extremely easy. – Etherege

@JonSkeet: That's great, but I have a similar case where the property I need to evaluate is determined at run time by the user. So I tried Expression<Func<Hotel, decimal>> getProperty = h => h.NumberOfDiamonds; and then query = query.Where(h => getProperty(h) == 0) but I get the error getProperty is a variable but is used like a method. Is that possible? – Heyduck

Answer 2

I was getting this very error and I'm using Entity Framework with PredicateBuilder by Joe Albahari to build dynamic where clauses. If you happen to be in the same condition, you should call the AsExpandable method:

If querying with Entity Framework, change the last line to this:

return objectContext.Products.AsExpandable().Where(predicate);

This method is part of LINQKIT DLL that you can grab here or through a NuGet package here.

Everything works fine now. :)

Underdog

Do you know if AsExpandable() causes any performance impact? I am using a generic repository and only use the predicate builder occasionally, is it worth it to create a separate repository method for predicate building? – Huey

@Cody... I have no idea. You'd better ask that to Joe Albahary directly. :) This is his twitter: twitter.com/linqpad – Underdog

@DoctorOreo Have you figured out the performance impact? I'm in the same situation as you – Histone

@Histone you know what, I've been using it since I guess April of 2014 and I haven't noticed any issues. Going on a year and a half now in a production application, nobody has complained. (this is a system with ~50 users dealing with anywhere from 10,000 to 5,000,000 records) – Huey

Looks like with the latest version you don't need to use AsExpandable() to pass in the predicate to Where(). Possibly because of the new PredicateBuilder.New<T>() starting point? – Bloodyminded

Answer 3

You can call the Expand() method on your predicate before the Where request.

Scarlet

This works too ! And I don't know what AsExpandable do, so I prefer to just call Expand when I really want to filter the query. – Domenic

Answer 4

I know this answer is really late, but I ran into the same problem and it led me here, so I thought I'd share my solution.

I read Leniel's answer, and it gave me an idea. The default types have the method "AsEnumerable()" which behaves the same way, alleviating the issue.

Beedon

AsEnumerable() will cause the Where() function to be invoked by Linq to Objects and not Linq to Entities. – Ransdell