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


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


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Mohsen Sarkar
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Mohsen Sarkar

Low level coder who like Flutter related technologies.

Updated on August 09, 2020


  • Mohsen Sarkar
    Mohsen Sarkar about 2 years

    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?

    • Lucas Trzesniewski
      Lucas Trzesniewski over 7 years
      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
      xanatos over 7 years
      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 :-)