Pre defined expressions in LINQ query sytax

Question

Is it possible to use a previously defined expression in LINQ expressions with query syntax? I've created a contrived example to illustrate my question.

var tabletTypes = new List<int> { 2, 3 };
//I want to use this expression many times
Expression<Func<Computer, bool>> isTablet = comp => tabletTypes.Contains(comp.Type);

var computers = db.Computers;
var producers = db.Producers;

//I know how to use the expression in fluent syntax
IQueryable<Producer> tabletProducers = computers
                               .Where(isTablet)
                               .Join(producers, 
                                     comp => comp.ProducerId, 
                                     producer => producer.Id, 
                                     (comp, producer) => producer);

//How can I use the expression in query syntax
IQueryable<Producer> tabletProducers2 = from c in computers
                                        join p in producers
                                            on c.ProducerId equals p.Id
                                        where /*How can I use isTablet here?*/
                                        select p;


public class Producer
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Computer
{
    public int Id { get;set; }
    public int Type { get; set; }
    public int ProducerId { get; set; }
}

I am using C# 5 and EF 5.

You can do that if you work with the expression tree. Have a look at [LinqKit](http://www.albahari.com/nutshell/linqkit.aspx), especially the `Expand()` functionality. — Simon Belanger, Jul 05 '13 at 17:03
@SimonBelanger I've never used LinqKit. It looks interesting. — Steven Wexler, Jul 05 '13 at 17:14

score 0 · Answer 1 · answered Jul 05 '13 at 16:09

0

You can just use where isTablet.Compile()(c). You might want to cache the compiled version.

answered Jul 05 '13 at 16:09

Rob

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Using this produces an Entity Framework error "The LINQ expression node type 'Invoke' is not supported in LINQ to Entities." – Steven Wexler Jul 05 '13 at 16:44
To explain my comment a bit further...this answer would work with `IEnumerables`, but not `IQueryables`. – Steven Wexler Jul 05 '13 at 16:53

Pre defined expressions in LINQ query sytax

1 Answers1