Casting to a derived type in a LINQ to Entities query with Table Per Hierarchy inheritance
.net c# entity-framework linq linq-to-entities
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();
Accepted Answer
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();
Popular Answer
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
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