XamlTaskFactory, CodeTaskFactory and RoslynTaskFactory tasks should work out of process.

[AR-May]

Tasks created using XamlTaskFactory, CodeTaskFactory, and RoslynTaskFactory currently do not support out-of-process execution. This limitation prevents these tasks from running in isolated task host processes, which is necessary for single-node, multithreaded build scenarios.

[JanProvaznik]

why is it necessary?

[AR-May]

If a customer had written any task that is not a new (thread-safe) Task, it should be offloaded to the task host process in the single-process scenarios. This includes tasks written with those task factories.

[JanProvaznik]

option 1: enlighten the taskfactory to compile the task out of proc and and ensure the taskhost can use it
option 2: compile to a file instead of in memory and pass that file's path to the taskhost it can then create the instance of the task (this is how it happens for AssemblyTaskFactory tasks)