Inconsistent handling of nulls about protobuf-csharp-port HOT 2 CLOSED

I believe that you are asking the wrong question.  I don't think that the cast 
in the C# implementation should be done differently.  To mimic the Java 
behavior, you simply need to change the line:

if (castMessage == null) {

to

if (castMessage == null && message != null) {

The C# implementation will then match the behavior of the Java implementation 
exactly.

I don't expect to do anything on this. We might revisit it when we look at the 
generated code for 3.0, but I suspect we won't look into it *specifically* - it 
may just drop out.