问题
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.
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);
The error :
Error CS8072 An expression tree lambda may not contain a null propagating operator.
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);
I'm curious why C# does nothing and simply throws a compiler error?
回答1:
It's complicated since expression tree lambdas (unlike delegate lambdas) are interpreted by already existing LINQ providers which don't yet support null propagating.
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
You can go deeper in the relevant discussion on CodePlex where 3 solutions are offered: NullPropagationExpression, ConditionalExpression & a hybrid
来源：https://stackoverflow.com/questions/28880025/why-cant-i-use-the-null-propagation-operator-in-lambda-expressions