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

问题

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();

回答1:

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();

回答2:

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();

回答3:

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();

回答4:

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?

回答5:

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

来源：https://stackoverflow.com/questions/12165805/casting-to-a-derived-type-in-a-linq-to-entities-query-with-table-per-hierarchy-i

标签

.net

linq

entity-framework

linq-to-entities