[Revit API] Simple Modeless Form (External Event Handler) in pyRevit

Warning : Some change in Revit API and Revit behaviour makes this article obsolete with newer version of Revit. I advise you to install pyRevitMEP extension and study updated code : FormExternalEventHandler.pushbutton. For usage in your script I recommend to use my CustomizableEvent from pyRevitMEP library.

I struggled for a while to make a modeless form. Why did I need it ? Because each time I was trying to get user to select object after WPF appear I was going out of Revit API thread and got this very common exception «Autodesk.Revit.Exceptions.InvalidOperationException: Attempting to create an ExternalEvent outside of a standard API execution». As Jeremy Tammik says :

One of the most frequently raised questions around the Revit API is how to drive Revit from outside, e.g., from a separate thread, a modeless dialogue, or a stand-alone executable.

I have read many examples on the subject. Most on them were in C#.

So I made a very simple form to make a very simple ExternalEventHandler sample as pyRevit script. It will help me and I hope it will help some hackers to struggle less than I did.

Let’s start with common import statement using built-in pyRevit utils :

# noinspection PyUnresolvedReferences
from Autodesk.Revit.UI import IExternalEventHandler, ExternalEvent
# noinspection PyUnresolvedReferences
from Autodesk.Revit.DB import Transaction
# noinspection PyUnresolvedReferences
from Autodesk.Revit.Exceptions import InvalidOperationException
from revitutils import selection, uidoc, doc
from scriptutils.userinput import WPFWindow

__doc__ = "A simple modeless form sample"
__title__ = "Modeless Form"
__author__ = "Cyril Waechter"

Then let’s write a simple function we want to execute modeless  (here it just delete selected elements) :

# Simple function we want to run
def delete_elements():
    t = Transaction(doc, "Failing script")
    for elid in uidoc.Selection.GetElementIds():
        print elid

And now come the new magic thing that let you enter in a valid Revit API context. The «ExternalEvent» class with his «IExternalEventHandler» class :

# Create a subclass of IExternalEventHandler
class SimpleEventHandler(IExternalEventHandler):
    Simple IExternalEventHandler sample

    # __init__ is used to make function from outside of the class to be executed by the handler. \
    # Instructions could be simply written under Execute method only
    def __init__(self, do_this):
        self.do_this = do_this

    # Execute method run in Revit API environment.
    def Execute(self, uiapp):
        except InvalidOperationException:
            # If you don't catch this exeption Revit may crash.
            print "InvalidOperationException catched"

    def GetName(self):
        return "simple function executed by an IExternalEventHandler in a Form"

# Now we need to make an instance of this handler. Moreover, it shows that the same class could be used to for
# different functions using different handler class instances
simple_event_handler = SimpleEventHandler(delete_elements)
# We now need to create the ExternalEvent
ext_event = ExternalEvent.Create(simple_event_handler)

Let’s do a simple form so easily created thanks to pyRevit in order to use our new toy :

# A simple WPF form used to call the ExternalEvent
class ModelessForm(WPFWindow):
    Simple modeless form sample

    def __init__(self, xaml_file_name):
        WPFWindow.__init__(self, xaml_file_name)
        self.simple_text.Text = "Hello World"

    def delete_click(self, sender, e):
        # This Raise() method launch a signal to Revit to tell him you want to do something in the API context

# Let's launch our beautiful and useful form !
modeless_form = ModelessForm("ModelessForm.xaml")

Here is the xaml code :

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 Title="Delete things:" Height="150" Width="300" ShowInTaskbar="False" Topmost="True"
 WindowStartupLocation="CenterScreen" ScrollViewer.VerticalScrollBarVisibility="Disabled" HorizontalContentAlignment="Center">
 <StackPanel Margin="20" HorizontalAlignment="Stretch">
 <TextBlock x:Name="simple_text" Text="" Grid.Column="0" HorizontalAlignment="Center" FontWeight="Bold"/>
 <Button Content="Delete selected elements" Height="30" Margin="10,10" Click="delete_click"/>

Thanks a lot to all people mentioned in this article and linked article and stuffs.


  1. where should i input those scripts ? The RevitPythonShell can not import from revitutils import selection, uidoc, doc and from scriptutils.userinput import WPFWindow ?

  2. I’m working in c#, but I just wanted to write you and tell you how refreshing it was to read a simple, concise example of how to do this. The sample codes on the forum and autodesk website (and the SDK) are horrendously overcomplicated. If it wasn’t for this post, I would have just given up. Thanks for posting this.

  3. You are legend Cyril.! I’ve been jumping backwards and forwards between the examples you mentioned at the very top of your post.. Couldn’t understand the whole thing at all.. Found your blog and this particular post. Following your example in RPS rather than in pyRevit it all worked. Magic 😉 thanks heaps!

  4. Thank you for this very useful article!

    I am trying to create a WPF which waits for an input from the WPF before executing the external event.

    E.G a checkbox boolean determines external placement

    Would you have a similar simple example of such a thing?

    I have dug around the internet but have had no luck.

    Thank you again.


  5. Hi there. Very sorry to bother you.

    I had a working implementation of this some years ago. I’ve recently come across the need for a modeless form, and it doesn’t seem to be working any longer with pyRevit scripts.

    I stumbled upon this article, and your implementation is very close to mine, so I copied this over, and this also does not work (Revit 2020, pyRevit 4.7.6). As strange as it seems, the modeless window seems to lose the ability to reference the global variables declared at the module level.

    Any thoughts?

    1. Hi,
      I added a warning message on top of the article :

      Warning : Some change in Revit API and Revit behaviour makes this article obsolete with newer version of Revit. I advise you to install pyRevitMEP extension and study updated code : FormExternalEventHandler.pushbutton. For usage in your script I recommend to use my CustomizableEvent from pyRevitMEP library.

      It works with pyRevit 4.7.6. If it doesn’t help, try to deactivate rocket mode and tell me if it works else, please open an issue on github.

      Also for use in a pyRevit script. Do not forget to add __persistentengine__ = True as done in samples.

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