可以在调用Where中调用named方法吗？ | 可以在调用Where中调用named方法吗

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问题描述

我试图从RedGate
，以便正确地转换呼叫。

I am trying to understand some performance implication of Linq from this free ebook by RedGateftp://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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