可以在调用Where中调用named方法吗？ [英] Is it possible to call named method within a call to Where?

问题描述

我试图从RedGate
ftp://support.red-gate.com/ebooks/under-the-hood-of-net-memory-management-part1.pdf

在本书第157-158页中，他们创建了以下示例。

  Order [] pastDueAccounts = null;日期时间
 using（varcontext = new Context（））
 {
 pastDueAccounts = context.Accounts.Where（account => account.DueDate< dueDate）.ToArray（）; 
} 
  

然后，他们将lamda表达式的一部分重新定义为以下函数。 >

  public bool PastDueAccount（Account account）
 {
 return account.DueDate< DateTime.Today.AddDays（-7）; 
} 
  

最后他们使用这个功能如下。

  Order [] pastDueAccounts = null; 
 using（varcontext = new Context（））
 {
 pastDueAccounts = context.Accounts.Where（account => PastDueAccount（account））ToArray（）; 
} 
  

根据我迄今所研究的内容，无法运行此linq查询因为LINQ将无法识别该方法，并且无法将其转换为存储表达式。我不知道这个例子是错误的，根本不可能运行，或者如果我很难听到关于如何模拟这个问题的话。

解决方案

您是正确的，LINQ到实体不能以显示的方式调用。

在LINQ到实体中使用的是：

• 在服务器端创建等效的功能。


• 适当地映射模型中的功能，以便正确地转换呼叫。

I am trying to understand some performance implication of Linq from this free ebook by RedGate ftp://support.red-gate.com/ebooks/under-the-hood-of-net-memory-management-part1.pdf

On page 157-158 in this book, they created following example.

Order[] pastDueAccounts = null;  
DateTimedueDate = DateTime.Today.AddDays(-7);  
using(varcontext = new Context())    
{  
    pastDueAccounts = context.Accounts.Where(account => account.DueDate < dueDate).ToArray();  
} 

They then re-factored part of lamda expression into following function.

public bool PastDueAccount(Account account)  
{  
    return account.DueDate < DateTime.Today.AddDays(-7);  
} 

Finally they used this function as follows.

Order[] pastDueAccounts = null;  
using(varcontext = new Context())  
{  
    pastDueAccounts = context.Accounts.Where(account => PastDueAccount(account)).ToArray();  
} 

Based on what I researched so far, its not possible to run this linq query as LINQ will not be able recognize the method and cannot be translate into a store expression. I wonder if this example is wrong and simply not possible to run or if I am just having hard time getting my heard around on how to simulate this problem?

解决方案

You are correct, this would not be able to be called by LINQ-to-Entities the way it's displayed.

The only way it could be used in LINQ-to-Entities is to:

• Create the equivalent of the function on the server side.
• Map the function in the model appropriately so that it translates the call correctly.

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