C#: Minor extractor fixes

[calumgrant]

This fixes two small issues that were causing messages in csharp.log.

1. Local variables were always expecting to be used in a "tagged" context, but this failed when they were declared in lambdas in fields properties. Change this to "optionally tagged" instead.
2. The dynamic typename was not extracted.

[hvitved]

One question, otherwise LGTM.

[hvitved]

Can you elaborate on why this is needed? I wonder because the Field entity sets TrapStackBehaviour.PushLabel, which I assume happens before extracting the initializer?

[calumgrant]

The reason is because fields don't use the tag stack. Most of the time, local variables are declared inside methods, or in lambdas in methods, so the tag stack is non-empty. However, a local variable can be declared inside a lambda which is used as an initializer to a field, e.g. here.

In those cases, there is no surrounding method, and therefore the tagstack is empty. This was causing the tagstack unexpectedly empty messages in csharp.log.

Changing the behaviour to OptionalLabel removes that warning.

[hvitved]

The reason is because fields don't use the tag stack

Then what does my link from above mean, I must have misunderstood something? (https://github.com/Semmle/ql/blob/master/csharp/extractor/Semmle.Extraction.CSharp/Entities/Field.cs#L99)

[calumgrant]

I reverted this change because it was the wrong fix. It actually turned out that the erroneous case was a property with an expression body, so I changed the trap stack behaviour of properties to PushesLabel.

[hvitved]

Excellent, thanks for investigating this!

[jf205]

change note LGTM