Accessing the Revit API in Callbacks from Other Threads

Georg Dangl by Georg Dangl in DotNet Tuesday, January 21, 2020

Tuesday, January 21, 2020

Posted in BIM Revit

Just this week, I've been doing some work with a plugin for Autodesk Revit. The general idea of the plugin is to show a window which basically just wraps a Chromium embedded browser to display a web UI and interact with the Revit API. The interaction itself is pretty straightforward - with CefSharp, you can bind JavaScript objects in the browser to .NET objects in the plugin and thus access Revit features.

We've implemented that by having a singleton on the .NET side that listens to commands from the browser. The first, possibly naive approach, was to simply call our code whenever we got a message:

However, this didn't work. While all the data was transferred correctly, whenever we accessed certain features of the Revit API, we got a SEHException from unmanaged code in Revit. A quick Google search yielded this bit of info: "SEHExceptions are always indications of a bug in Revit.". Ok, we thought, but that's super basic stuff we're accessing, there's probably not an undiscovered bug with Revit here.

Luckily, debugging this gave us a bit more info: The API is working correctly when accessed from the main thread, but throws when accessed by another one. So even though we weren't doing any UI operations, this seemed to be the root cause of our issue. Since you also can't invoke something on the main thread in Revit, with Application.Current.Dispatcher not being set, we fell back to using the UIApplication.Idling event and a stack to invoke our methods on the main thread:

This cost us a few hours, so I hope it's helpful to others!

Happy integrating!

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Hi, my name's George! I love coding and blogging about it. I focus on all things around .Net, Web Development and DevOps.


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