转换为派生类中的LINQ to实体表每层次继承查询 [英] Casting to a derived type in a LINQ to Entities query with Table Per Hierarchy inheritance

问题描述

我有一个LINQ到实体模型表每层次继承。我有过基本类型的查询，而我想要做的具体类型相关的逻辑。例如：

 的IQueryable＆LT;基本类型＆GT;基地= ...

//这工作正常
变种结果= base.Select（二=＆GT; b为DerivedType1？1：2）.ToList（）;

//这不会编译到SQL
VAR结果2 = base.Select（B =＆GT; b为DerivedType1（（DerivedType1）B）.DerivedProperty：空）.ToList（）;
 

有没有办法做这样的事情，而不用单独处理每个派生类型的IQueryables：

  //我宁愿不这样做：
VAR resultA = base.OfType＆LT; D​​erivedType1＆GT;（）选择（D =＆GT; d.DerivedProperty）;
VAR resultB = base.OfType＆LT; D​​erivedType2＆GT;（）选择（D =＆GT;默认值（INT）？）;
VAR的结果= resultA.Concat（resultB）.ToList（）;
 

解决方案

直接铸像实体类型（DerivedType1）乙不支持使用LINQ于─实体但为操作符（ B为DerivedType1 ）的，因此你可以尝试：

  VAR结果2 =基地
    。选择（B =＆GT; b为DerivedType1
        ？ （二为DerivedType1）.DerivedProperty
        ： 空值）
    .ToList（）;
 

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();

解决方案

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();

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