从dotnet Core 2.2.6更改为3.0.0后的EF Linq错误

问题描述

我正在尝试将解决方案升级到新的Core Framework 3.0.0。
现在我有一个我不明白的小问题。

I'm trying to upgrade a solution to the new Core Framework 3.0.0.Now I'm having a small issue I don't understand.

看，此方法在2.2.6中没有问题：

Look, this method was unproblematic in 2.2.6:

public async Task<IEnumerable<ApplicationUser>> GetBirthdayUsersCurrentMonth()
    {
        return await ApplicationDbContext.Users
            .Where(x => x.Gender != ApplicationUser.GenderTypes.generic)
            .Where(x => x.BirthDate.GetValueOrDefault().Month == DateTime.Now.Month)
            .Where(x => x.RetireDate == null)
            .OrderBy(x => x.BirthDate.GetValueOrDefault())
            .ToListAsync();
    }

现在在3.0.0版本中，我收到一个Linq错误，说：

Now in 3.0.0 I get a Linq Error saying this:

当我禁用此行时：

.Where(x => x.BirthDate.GetValueOrDefault().Month == DateTime.Now.Month)

错误消失了但是当然，我得到了所有用户。而且我在此查询中看不到错误。
这可能是EF Core 3.0.0中的错误吗？

The error is gone but off course I get all users. And I can't see an error in this query.Could this perhaps be a bug in EF Core 3.0.0?

推荐答案

原因是隐式客户端评估具有已在EF Core 3中被禁用。

The reason is that implicit client evaluation has been disabled in EF Core 3.

这意味着以前，您的代码未在服务器上执行WHERE子句。取而代之的是，EF将所有行加载到内存中并评估内存中的表达式。

What that means is that previously, your code didn't execute the WHERE clause on the server. Instead, EF loaded all rows into memory and evaluated the expression in memory.

要在升级后解决此问题，首先，您需要弄清楚EF可以做什么？ t转换为SQL。我的猜测是对 GetValueOrDefault（）的调用，因此请尝试像这样重写它：

To fix this issue after the upgrade, first, you need to figure out what exactly EF can't translate to SQL. My guess would be the call to GetValueOrDefault(), therefore try rewriting it like this:

.Where(x => x.BirthDate != null && x.BirthDate.Value.Month == DateTime.Now.Month)

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