.NET: Is JsonMediaTypeFormatter thread safe?

I believe the automatically-generated documentation just uses that common phrase for most classes.

Serializers/formatters are usually designed in a way they can be reused from different threads. The thread-safety criteria is they don't update any instance variables in an unsafe manner. And you can always ensure it with ildasm, ilspy or any other .net decompiler tool.

For example, your JsonMediaTypeFormatter.WriteToStreamAsync() only reads instance properties and calls static methods. The only exception is _dataContractSerializerCache.GetOrAdd() call inside GetDataContractSerializer() (only when UseDataContractJsonSerializer is enabled), but it is also safe since _dataContractSerializerCache is a ConcurrentDictionary, hence thread-safe by design.

So there's no reason for JsonMediaTypeFormatter to be thread-unsafe.

P.S. On the other hand, I don't see how caching JsonMediaTypeFormatter will improve performance a lot, because there's no heavy work done inside the constructor, just some fields' initialization. Compared to serialization itself and network I/O overhead, it should be quite negligible.

After I struggled to find what was causing performance issues in my code, I did a gist to prove where the performance issue was:

https://gist.github.com/anonymous/6c47b4138595e13f53c162d1c69ba0ed(A copy-n-paste made the gist use the static method two times in the last tuple).

The problem is in the code produced to read/write using DataContractJsonSerializer.

Also, I didn't find any official documentation regarding this issue neither in Mictosoft documentation nor in the internet.

Thanks for pointing us up in the right direction. I don't know if it's possible, but you could edit the question title and add performance issues in it.