为什么我不能使用lambda表达式零传播经营者？ [英] Why can't I use the null propagation operator in lambda expressions?

问题描述

我经常使用空传播经营者在我的代码，因为它给了我更多的可读的代码，特别是在长的查询，我没有为null检查所使用的每一个类。

I often use null propagating operator in my code because it gives me more readable code, specially in long queries I don't have to null-check every single class that is used.

下面的代码抛出，我们不能在拉姆达使用null传播运营编译错误。

The following code throws a compile error that we can't use null propagating operator in lambda.

var cnt = humans.AsQueryable().Count(a => a.House?[0].Price == 5000);

错误：

The error :

错误CS8072表达式树lambda不能包含null传播运营商。

Error CS8072 An expression tree lambda may not contain a null propagating operator.

C＃可以很容易地转换上面的代码的代码以下面的代码，如果真的不能做任何事情！

C# Could easily translate above code to the code to following code if really can't do anything else!

var cnt = humans.AsQueryable().Count(a => a.House != null && a.House[0].Price == 5000);

我很好奇，为什么C＃什么也不做，只是抛出一个编译器错误？

I'm curious why C# does nothing and simply throws a compiler error?

推荐答案

由于表达式树lambda表达式（不像委托lambda表达式）通过现有的LINQ提供解释这还不支持空传播很复杂。

It's complicated since expression tree lambdas (unlike delegate lambdas) are interpreted by already existing LINQ providers which don't yet support null propagating.

转换为条件表达式，因为有多种评估并不总是准确的，而与 这里只有例如一个评价？：

Converting to a conditional expression is not always accurate as there are multiple evaluations while with ?. there's only a single evaluation for example:

customer.Where(a => c.Increment()?.Name) // Written by the user 
customer.Where(a => c.Increment() == null ? null : c.Increment().Name) // Incorrectly interpreted by an old LINQ provider

您可以在CodePlex上相关讨论走向深入其中，3解决方案可供选择： NullPropagationExpression ，条件表达式&安培;混合

You can go deeper in the relevant discussion on CodePlex where 3 solutions are offered: NullPropagationExpression, ConditionalExpression & a hybrid

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