为什么LINQ-to-Entites会识别我的自定义方法？ [英] Why does LINQ-to-Entites recognize my custom method?

问题描述

这样做：

  Entities.WorkOrderSet.Where（MyCustomMethod）; 
  

这不是：

  Entities.WorkOrderSet.Where（o => MyCustomMethod（o））; 
  

（ 即使没有新，它不工作）

我明白为什么第二个不起作用 - 但为什么在世界上做第一个工作！？不应该在运行时得到一个LINQ到实体不能识别方法，就像第二个？ / p>

为了参考，这里是MyCustomMethod

  public bool MyCustomMethod（WorkOrder workOrder ）
 {
 return！workOrder.WorkOrderNum.StartsWith（A，StringComparison.CurrentCultureIgnoreCase）; 
} 
  

使用EF1，而不是EF4

解决方案

首先作品是因为是一种扩展方法，正在将查询作为func执行，然后过滤列表参见这里。
所以一般来说，它将自动转换到

  Where（Func< WorkOrder，bool> 
  

第二个不是因为它将你的where语句推送到db当lambda表达式被评估时，它被展开像这样：

 其中（Expresion< Func< WorkOrder，bool>>）
  

这是一个很好的表达式 vsnofollow noreferrer>文章 Func

这是另一个SO帖子，有助于解释差异

这个新的编辑看起来是正确的。澄清：似乎 Func< WorkOrd呃，bool> 隐含地转换为表达式< Func< WorkOrder，bool>> ，而不是其他方式。

这两种类型都存在 Where 的重载。 。（MyCustomMethod）正在调用 Func< WorkOrder，bool> 一个，而。其中（o => MyCustomMethod（o））正在调用 Expression&FunC< WorkOrder，bool>>

This works:

Entities.WorkOrderSet.Where(MyCustomMethod);

This does not:

Entities.WorkOrderSet.Where(o => MyCustomMethod(o));

([Edit] Even without new, it doesn't work)

I understand why the second doesn't work - but why in the world does the first work!? Shouldn't I get a "LINQ-to-Entities does not recognize the method..." at runtime, like with the second?

For reference, here is MyCustomMethod

public bool MyCustomMethod(WorkOrder workOrder)
{
    return !workOrder.WorkOrderNum.StartsWith("A", StringComparison.CurrentCultureIgnoreCase);
}

Using EF1, not EF4

解决方案

First works because it is an extension method and is is executing the query as a func, and then filtering your list see here. So in general it would automatically cast the where to

 Where(Func<WorkOrder, bool>

Second doesn't because it is pushing your where statement down to the db. When the lambda expression is evaluated it is expanded like this:

Where( Expresion<Func<WorkOrder, bool>>)

Here is a good article that explains Expressions vs Func

Here is another SO post that helps to explain the difference

[Edit (BlueRaja)]

This new edit appears to be correct. To clarify: it seems Func<WorkOrder, bool> is implicitly castable to Expression<Func<WorkOrder, bool>>, but not the other way around.

There are overloads of Where for both types. .Where(MyCustomMethod) is calling the Func<WorkOrder, bool> one, whereas .Where(o => MyCustomMethod(o)) is calling the Expression<Func<WorkOrder, bool>> one.

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