Why can't I use the null propagation operator in lambda expressions?

Question

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?

Solution

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

Comments

[Lucas Trzesniewski]

Foo?.Bar is not equivalent to Foo != null ? Foo.Bar : null because Foo is evaluated once with the null-propagating operator, and twice with the conditional, so the translation wouldn't be correct in all cases.

[xanatos]

Note that if its code for EF, there is the possibility that you don't really need the null propagating operator, because when a query is converted to SQL call, SQL doesn't throw nulls :-)