New Post: FuntionFilter inside of a custom Tree Definition not working, want am I doing wrong?

It looks like the FunctionFilter doesn't support External C# functions.

The issue is that our External C# Function feature don't handle static classes - kind of silly (and I'm logging a bug on it as I write this). The method needs to be static, but the class must not. With those changes you should see things work just fine. The reason you did not see your external c# function work as a function filter was because it was not working at all.