c# - 表达式不能包含 lambda 表达式

Question

我已经获取了 List<>对象如下(带有 .Include() ):

List<vDetail> entityvDetails =
    context.vDetails
    .Include("payInstallment.appsDetail")
    .Include("payInstallment.appsDetail.application")
    .Include("payInstallment.appsDetail.purposes")
    .Where(e => e.vch_id == 123).ToList();

然后在前面代码的某处，我尝试按如下方式过滤实体记录:

foreach (vDetail item in lstVDetails)
{
    ... 

    int purposeId = entityvDetails.Where(e => e.sad_id == item.sad_id).FirstOrDefault().payInstallment.appsDetail.purposes.prp_id;

    ...
}

代码编译完美。但是，运行时返回以下错误(尽管包括所有导航):

Object reference not set to an instance of an object.

所以我设置了使用监 window 口进行调试。现在在观察窗口中分析以下语句:

entityVoucherDetails.Where(e => e.sad_id == item.sad_id).FirstOrDefault()

监 window 口生成以下错误:

Expression cannot contain lambda expressions.

谁能告诉我是什么原因？

Answer 1

尚不支持在调试器(监 window 口)中评估 Lambda 表达式。

有一个开放的feature request

为了调试您的问题，您应该将 lambda 表达式的结果分配给专用变量并在以下语句中使用它。

var entity = entityvDetails.Where(e => e.sad_id == item.sad_id).FirstOrDefault();

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08/2014 更新: Microsoft 发布了功能请求的更新，宣布他们开始处理它:

So here’s where we stand.

• 1) We want this to work as much as you do. It’s not under review – it’s in progress.
• 2) We figured out how to make it work; it simply requires rewriting everything.
• 3) We’re rewriting everything.
• 4) Rewriting everything takes a lot of time and a lot of testing.

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更新 11/2014: 微软最终在 VS2015 中实现了一些限制。 Read here.