Python .NET, multithreading and the windows event loop

I have found answers to most of my own questions.

• I think we can't assume the DLL is thread safe, so probably best to isolate all interaction with the DLL to one thread.

• It looks like the Windows message system is per thread and not per process, so yes, every thread using the Windows message system is required to have a Windows message processing loop.

• One can insert execution in the a windows event loop using Form.Invoke. running a non-ui main loop, you can get a dispatcher using Dispatcher.CurrentDispatcher which can used for 'invoking' a function. The invoked function is then executed on the thread where the dispatcher is created. The called function needs to be delegated though, which is a C# specific thing to pass references to functions.

in the end I did something like this for a non-ui main loop:

import clr
import threading

# we need to get access to the threading assembly
clr.AddReference("c:\\Program Files\\Reference Assemblies\\Microsoft\\Framework\\v3.0\\WindowsBase.dll")

import System
from System.Windows.Threading import Dispatcher
from System.Windows.Forms import Application

# now get the dispatcher for the current thread
dispatcher = Dispatcher.CurrentDispatcher

def display():
    print(threading.current_thread())

# now make this a deligate
deligate = System.Action(display)

def other_thread(dispatcher):
    # now you can use the dispatcher from the main thread to
    # to schedule a run from an other thread
    dispatcher.Invoke(deligate)

thread = threading.Thread(target=other_thread, args=(dispatcher,))
thread.start()
Application.Run()

some related links:

• more info about the Dispatcher
• more about the Action delegate

I have had a similar problem: using a pythonnet library DLL (import clr, et.) from multiple Python threads at the same time.

I had errors when multiple threads were communicating with the same functions (managing a hardware device) in parallel.

Solution:

1. create a global lock:

   lock = threading.Lock()
   

2. replace each API call e.g. device.move(x=1234) by:

   with lock:
       device.move(x=1234)