foo.cs

using System;

interface ITreeNode { }
interface IScope { }

class C
{
    public bool InteriorShouldBeProcessed(ITreeNode element)
    {
      return !(element is IScope) || ScopeShouldBeVisited((IScope) element);
    }
    
    public bool InteriorShouldBeProcessed2(ITreeNode element)
    {
      return !(element is IScope s) || ScopeShouldBeVisited(s);
    }
    
    public bool InteriorShouldBeProcessed3(ITreeNode element)
    {
      return !(element is IScope s && !ScopeShouldBeVisited(s));
    }
    
    public bool ScopeShouldBeVisited(IScope s) => true;
}