Casting to a derived type in a LINQ to Entities query with Table Per Hierarchy inheritance

Question

I have a LINQ to entities model with Table Per Hierarchy inheritance. I have a query over the base type, and I want to do specific type-dependent logic. For example:

IQueryable<BaseType> base = ...

// this works fine
var result = base.Select(b => b is DerivedType1 ? 1 : 2).ToList();

// this doesn't compile to SQL
var result2 = base.Select(b => b is DerivedType1 ? ((DerivedType1)b).DerivedProperty : null).ToList();

Is there any way to do something like this without processing IQueryables of each derived type separately:

// I'd rather not do this:
var resultA = base.OfType<DerivedType1>().Select(d => d.DerivedProperty);
var resultB = base.OfType<DerivedType2>().Select(d => default(int?));
var result = resultA.Concat(resultB).ToList();

Could the problem be that your DerivedProperty isnt nullable? Then could the query be confused as to whether the column allowed nulls? — Kirsten, Mar 26 '13 at 12:53

score 31 · Accepted Answer · answered Apr 18 '13 at 16:49

31

Direct casting to an entity type like (DerivedType1)b isn't supported with LINQ-to-Entities but the as operator (b as DerivedType1) is, hence you could try:

var result2 = base
    .Select(b => b is DerivedType1
        ? (b as DerivedType1).DerivedProperty
        : null)
    .ToList();

answered Apr 18 '13 at 16:49

Slauma

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This does seem to work, although I remember trying this before posting so maybe it only works in EF5+? – ChaseMedallion Jul 23 '13 at 19:32
3

@ChaseMedallion: Possibly, yes. I believe I've never used this before EF 5, but I'm not sure. BTW: In the meantime I discovered that if `DerivedProperty` is nullable the ternary operator is not needed, you can just use: `Select(b => (b as DerivedType1).DerivedProperty)`. If `b` is not of type `DerivedType1` EF won't throw an exception but just return `null` automatically. – Slauma Jul 23 '13 at 19:43
Right. EF does null-coalescing unless you are pulling a null into memory as a non-nullable type. – ChaseMedallion Jul 24 '13 at 19:22
1

@Slauma Adding to what you wrote: For non-nullable types, it appears like you can simply cast to nullable. For example, assuming `DerivedProperty` is an `int`: `Select(b => (int?) (b as DerivedType1).Derive‌dProperty)` – Zero3 Aug 17 '16 at 09:52

score 1 · Answer 2 · answered Apr 18 '13 at 13:49

OfType<DerivedType1>()

will return an IEnumerable, if possible, try to change to base-type to IEnumerable instead of IQueryable, you might en up in some SQL restrictions when using IQueryable.

That is of course if you are not actually quering a database?

score 1 · Answer 3 · edited Jul 23 '13 at 20:52

You can use EntityFramework.Extended to improve the performance of the query instead of doing 2 round trips to DB.

var resultA = base.OfType<DerivedType1>().Select(d => d.DerivedProperty).Future();
var resultB = base.OfType<DerivedType2>().Select(d => default(int?)).Future();
var result = resultA.Concat(resultB).ToList();

In this case only one round trip to bd is executed. This framework is very useful for many other things int EF

score 0 · Answer 4 · answered Aug 31 '12 at 11:55

You could have a method on your base type that's overridden in your derived types to provide the relevant property value.

public class MyBaseClass
{
    public virtual int GetSomething()
    {
        throw new NotImplementedException();
    }
}

public class MyDerivedClass1 : MyBaseClass
{
    public int SomeProperty { get; set; }

    public override int GetSomething()
    {
        return this.SomeProperty;
    }
}

public class MyDerivedClass2 : MyBaseClass
{
    public int SomeOtherProperty { get; set; }

    public override int GetSomething()
    {
        return this.SomeOtherProperty;
    }
}

Then you could:

var result = base.Select(b => b.GetSomething()).ToList();

This won't work with LINQ to Entities - it will require .ToList() first to bring entities into memory, which, since only one property per entity is required by query, is highly inefficient. — Danny Varod, Aug 31 '12 at 11:59

Joshua G · Answer 5 · 2013-01-09T21:46:11.107

0

Try this, I have never done anything with needing to do this kind of this but this should do it. Also if you use base, first of all don't because it is a keyword but if you must, use @base the @ in front of the name denotes that it is not used as a keyword.

var resultA = base.Select(aVar =>
                            (aVar is DerivedType1) ?
                                (DerivedType)(((DerivedType1)aVar).DerivedProperty)
                                :
                                (DerivedType)(default(int?))
                        ).ToList();

edited Jan 09 '13 at 21:46

answered Jan 09 '13 at 01:16

Joshua G

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Did you mean to answer a different question? This isn't an answer to my question... – ChaseMedallion Jan 09 '13 at 12:48

Casting to a derived type in a LINQ to Entities query with Table Per Hierarchy inheritance

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