More robust data structures

[doug-jacob]

In order to smooth over transitions from the old Transit framework to the new one, some changes have been implemented for data structures.

NullKeyDictionary now implements IDictionary<object, object> and IReadOnlyDictionary<object, object> to make it more similar to the type it replaces from the old Transit, System.Collections.Immutable.ImmutableDictionary<object, object>. Callsites that originally used the old Transit expected to be able to use dictionary-related methods and extensions that are not all available from the System.Collections.IDictionary interface. This allows said callsites to simply change the type they cast values read from Transit from ImmutableDictionary<object, object> to IDictionary<object, object> without needing to change any other logic.

A wrapper class ListWrapper has also been added to preserve serialization information. Lists we receive from environments like Clojure would normally most closely be represented by LinkedLists in C#, but we prefer to use List for performance. Therefore, this wrapper class is used so that if we need to roundtrip data back to the original environment, the original type of the list is maintained in the data we send. Similarly to the situation with dictionaries, the old Transit read lists as ImmutableList<object>, and so for simplicity and to minimize impact on callsites, the wrapper class inherits from List<object>.

[oconnor0]

Why null! when you're producing a value of object??

[doug-jacob]

Ah, whoops. This was copied verbatim from a few lines above, where we create a DictionaryEntry object for similar reasons, and the signature of the DictionaryEntry constructor takes a non-null object for a key

[oconnor0]

Ah, OK, was hoping it wasn't some required C# thing because of the context or generic arguments or something.

[oconnor0]

@isaksky, did you want to review this before I merge?

[isaksky]

It looks ok, but I see a test fails:

  Failed TestVerifyExemplarFiles [6 ms]
  Error Message:
   Assert.IsNotNull failed. Could not find transit-format exemplar directory
  Stack Trace:
     at Transit.Net.Tests.TransitTest.TestVerifyExemplarFiles() in /home/runner/work/transit-csharp/transit-csharp/src/Transit.Tests/TransitTest.cs:line 2265
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

[oconnor0]

Yeah, that's why #5 is still unmerged too.

[isaksky]

Ok. To answer the question more generally, no need to wait for me when you're ready to merge on this stuff.