Q:

Is it thread safe to use WriteAsync on NetworkStream from multyple threads at same time?

When having a TcpClient taking its stream in multiple threads and performing WriteAsync into it on each thread. Is it thread safe? Will data (byte arrays passed to write method) be delivered as intact chuncs passed to WriteAsync

A:

No, it is not safe.
From the MSDN under Remarks:

Read and write operations can be performed simultaneously on an
  instance of the NetworkStream class without the need for
  synchronization. As long as there is one unique thread for the write
  operations and one unique thread for the read operations, there will
  be no cross-interference between read and write threads and no
  synchronization is required.

So when you have multiple writing threads you need to synchronize these.

