LINQ to Entities方法未识别 - 扩展方法样式

Question

这里还有另外一个“LINQ to entities不能识别方法问题”......但是，下面的代码不是基本上完全相同的东西吗？

作品：

var returnData = from x in MyEntities.MyDBSet 
         where x.MyDBSetPrimaryKey == id 
         select new Models.MyModelDTO 
         { 
          MyPropOne = (int)x.MyModel.MyOtherPropOne, 
          MyPropTwo = x.MyOtherPropTwo ?? 0, 
          MyPropThree = x.MyModel.MyOtherPropThree, 
          MyPropFour = x.MyModel.MyOtherPropFour, 
          MyPropFive = x.MyModel.Entity.MyOtherPropFive, 
          MyPropSix = x.MyModel.MyOtherPropSix == null ? 0 : (decimal)x.MyModel.MyOtherPropSix, 
          MyPropSeven = x.MyModel.SomeType.MyOtherPropSeven, 
          MyPropEight = (int)x.MyModel.MyOtherPropEight, 
          MyPropNine = x.MyModel.MyPropNine == null ? 0 : (int)x.MyModel.MyOtherPropNine, 
          MyPropTen = x.MyModel.MyOtherPropTen == null ? 0 : (int)x.MyModel.MyOtherPropTen, 
          MyPropEleven = x.OtherEntity.MyOtherPropEleven, 
          MyPropTwelve = x.MyOtherpropTwelve 
         }; 

不起作用：

public static MyModelDTO ToModelDTO(this MyModel x) 
    { 
     return new MyModelDTO() 
     { 
      MyPropOne = (int) x.MyModel.MyOtherPropOne, 
      MyPropTwo = x.MyOtherPropTwo ?? 0, 
      MyPropThree = x.MyModel.MyOtherPropThree, 
      MyPropFour = x.MyModel.MyOtherPropFour, 
      MyPropFive = x.MyModel.Entity.MyOtherPropFive, 
      MyPropSix = x.MyModel.MyOtherPropSix == null ? 0 : (decimal) x.MyModel.MyOtherPropSix, 
      MyPropSeven = x.MyModel.SomeType.MyOtherPropSeven, 
      MyPropEight = (int) x.MyModel.MyOtherPropEight, 
      MyPropNine = x.MyModel.MyPropNine == null ? 0 : (int) x.MyModel.MyOtherPropNine, 
      MyPropTen = x.MyModel.MyOtherPropTen == null ? 0 : (int) x.MyModel.MyOtherPropTen, 
      MyPropEleven = x.OtherEntity.MyOtherPropEleven, 
      MyPropTwelve = x.MyOtherpropTwelve 
     }; 
    } 

后来打电话：

包裹在一个扩展方法完全相同的任务

var returnData = from x in MyEntities.MyDBSet 
         where x.MyDBSetPrimaryKey == id 
         select x.ToModelDto(); 

，导致：

LINQ to Entities does not recognize the method 'MyExtensionMethods.MyModels.MyModelDTO ToModelDTO(API.Models.MyModel)' method, and this method cannot be translated into a store expression. 

Answer 1

当查询供应商认为该方法不知道该怎么办。它不能进入​​并看到方法的源代码，它可以查看对象的方式并查看完成的操作。它不能在客户端评估它，因为它还没有这些项目，并且它不能想到任何SQL将该方法调用转换为。

相反，你应该写的是需要一个IQueryable并返回另一个IQueryable，像这样的方法：

public static IQueryable<MyModelDTO> ToModelDTO(this IQueryable<MyModel> query) 
{ 
    return query.Select(x => new MyModelDTO() 
    { 
     MyPropOne = (int)x.MyModel.MyOtherPropOne, 
     MyPropTwo = x.MyOtherPropTwo ?? 0, 
     MyPropThree = x.MyModel.MyOtherPropThree, 
     MyPropFour = x.MyModel.MyOtherPropFour, 
     MyPropFive = x.MyModel.Entity.MyOtherPropFive, 
     MyPropSix = x.MyModel.MyOtherPropSix == null ? 0 : (decimal)x.MyModel.MyOtherPropSix, 
     MyPropSeven = x.MyModel.SomeType.MyOtherPropSeven, 
     MyPropEight = (int)x.MyModel.MyOtherPropEight, 
     MyPropNine = x.MyModel.MyPropNine == null ? 0 : (int)x.MyModel.MyOtherPropNine, 
     MyPropTen = x.MyModel.MyOtherPropTen == null ? 0 : (int)x.MyModel.MyOtherPropTen, 
     MyPropEleven = x.OtherEntity.MyOtherPropEleven, 
     MyPropTwelve = x.MyOtherpropTwelve 
    }); 
} 

这里的映射仍然向下编译成一个Expression该查询供应商可以解析。现在，你可以这样做：

var returnData = (from x in MyEntities.MyDBSet 
        where x.MyDBSetPrimaryKey == id 
        select x) 
        .ToModelDTO(); 

Answer 2

您的问题和可能的解决方案是不是真的比许多其他“LINQ到实体无法识别方法”的问题有什么不同。

举例来说，看到https://stackoverflow.com/a/7259649/120955和https://stackoverflow.com/a/18901609/120955