[Revived] Null-propagating operator ?. in Expression Trees

[mkosieradzki]

I would like to revive and old from codeplex https://roslyn.codeplex.com/discussions/571077 and provide an implementation that does not required modifiaction from BCL but will be really handy for at least some scenarios (not all!).

I have implemented such an approach in my scripting engine basing on Roslyn (as frontend) and LINQ Expressions as a backend.

Expression.Invoke(Expression.Lambda(Expression.Condition(Expression.Equal(parm, Expression.Constant(null)), Expression.Default(rtype), translatedWhenNotNull), parm), translatedExpression);

It utilizes existing mechanism declaring inline lambda and invoking it. It preserves single-evaluation semantics, without requiring special support from BCL.

I am fully aware inline lambdas are actually blackboxes for many LINQ providers, but such a blackbox is still better than not supporting such a useful operator at all!

[svick]

The more general request of extending expression trees with features that are allowed in normal expressions is #2060. I think that it makes more sense to consider that proposal as a whole, instead of adding the features one by one.

[mkosieradzki]

@svick
I aggree. I was unable to find previous issue because I was searching for "null-propagating" as in original issue instead of "null-coalescing" ;). Thanks for pointing out proper issue.

[douglasg14b]

Correctly linking #2060 which is redirected to: dotnet/csharplang#2545