The calling thread must be STA

[jz499]

Environment

• Pythonnet version: 2.4
• Python version: 3.6.5
• Operating System: Windows 10

Details

Hi Everyone;
I'm trying to run a basic wpf application from within Python using VS code. When the script executes I get the error listed at the bottom of this page. Has anyone seen this error before?

My Code:

import clr
clr.AddReference("PresentationFramework")
clr.AddReference("PresentationCore")

from System.Windows import (Application, SizeToContent, Thickness, Window)
from System.Windows.Controls import (Button, Label, StackPanel)
from System.Windows.Media.Effects import DropShadowBitmapEffect

window = Window()
window.Title = 'Welcome'
window.SizeToContent = SizeToContent.Height
window.Width = 450

stack = StackPanel()
stack.Margin = Thickness(15)
window.Content = stack

button = Button()
button.Content = 'Button1'
button.FontSize = 24
button.BitmapEffect = DropShadowBitmapEffect()

def onClick(sender, event):
message = Label()
message.FontSize = 36
message.Content = 'Welcome'
stack.Children.Add(message)

button.Click += onClick
stack.Children.Add(button)

app = Application()
app.Run(window)

Error Output:

File "c:/Users/WIN-VMMV970AOGL/pyfolder/test.py", line 10, in
window = Window()
System.InvalidOperationException: The calling thread must be STA, because many UI components require this.
at System.Windows.Input.InputManager..ctor()
at System.Windows.Input.InputManager.GetCurrentInputManagerImpl()
at System.Windows.Input.KeyboardNavigation..ctor()
at System.Windows.FrameworkElement.FrameworkServices..ctor()
at System.Windows.FrameworkElement.EnsureFrameworkServices()
at System.Windows.FrameworkElement..ctor()
at System.Windows.Controls.Control..ctor()
at System.Windows.Window..ctor()

[lostmsu]

What you need is a single apartment thread for COM.

You can do it either by manually calling https://docs.microsoft.com/en-us/windows/win32/api/combaseapi/nf-combaseapi-coinitializeex or start a separate thread using .NET: https://stackoverflow.com/questions/11681666/starting-an-stathread-in-c-sharp

[lostmsu]

@jz499 I got more questions in email, but I don't see them in this thread. Have you been able to get it working? Should we close the topic?

[jz499]

Hi @lostmsu , I got it partially working. I created an apartment thread in Python, the code executes with no errors, but it does not display the window or controls. Here's the threading I added to the above code.

 thread = Thread(ThreadStart(window)
thread.SetApartmentState(ApartmentState.STA)
thread.Start
thread.Join

[lostmsu]

@jz499 I think that happens, because Python can only execute 1 thread at time. E.g. while main Python interpreter is busy in thread.Join(), the thread is unable to run any of your Python code in window.

Probably you need to call BeginAllowThreads somehow.

[jz499]

Hi @lostmsu,
Thanks, adding an apartment thread did resolve my problem. To make my code work I simply used a Xaml file rather than defining each control directly in python code.

stream = StreamReader("my_app.xaml")
window = XamlReader.Load(stream.BaseStream)
Application().Run(window)