Short Answer: Make sure all your assemblies are referencing the SAME EXACT JSON.NET DLL. What’s probably happening is you are applying [JsonProperty] from one DLL in one assembly, and serializing the object from a different assembly which is looking for a different [JsonProperty] and because the CLR object types are different it is effectively being ignored.
Longer Answer: I just had this problem but fortunately because I had one class that was working with JsonProperty and one that wasn’t I was able to do some detective work.
I stripped the non-working class down to the bare minimum and compared it to the working class and couldn’t see ANY differences – except for the fact that the non-working class was in a different assembly.
When I moved the class to the other assembly it worked perfectly as it should.
I poked around for a bit trying to look into JSON serialization of namespaces, but that didn’t seem to apply so I looked at the references and sure enough I was referencing an old JSONNET3.5 DLL in my entities DLL and the NUGET 4.5 version in my main project file.
This gives me two instances of [JsonProperty] attribute (which is just a regular class) and just because they’re named the same doesn’t mean the serializer is going to even recognise the attribute.